COSTO TOTAL DEL ELEVADOR DESMONTABLE

The com parison of the S group of opsins w ith 24 other vertebrate v isu al p ig m e n ts (Table 3.5) has p erm itted the identification of those residues w hich are exclusive to m em bers of the S group. For exam ple, Arg-67 is a chicken violet pigment-specific residue and is indicated in blue in Figure 3.8. O ther residues found only w ithin m em bers of the S group and not in any of the other visual pigm ents, at a particular position are indicated in blue in Figure 3.8. Examples of residues w hich are exclusive to m em bers of the S group are Trp-60 and Ala-120 (Figure 3.8). Trp-60 is found only in the chicken violet opsin, w hereas Val-60 and Phe-60 are p resen t in the other m em bers of the S group. Val-60 and Phe-60 are also found w ith in the other 24 opsins, and thus these residues are show n in p lain text in Figure 3.8. Ala-120 is found only w ithin 7 m em bers of the S group, w hereas Cys-120 is conserved in all m em bers of the L group and Gly-120 is fo u n d in all the other visual pigm ents including the chicken violet opsin (Figure 3.8).

R esidues w hich are identified as being exclusive to one or tw o m em bers of the S group can be used to follow the evolution of individual

R h group

Lam prey {Lamptera japonica)

Sand Goby (Pomatoschistus m in u tu s)

G oldfish {Carassius auratus)

Frog (Xenopus laevis)

C hicken (Gallus gallus)

H u m a n (Homo sapiens)

Cow (Bos tauras)

M ouse (M us musculus)

H am ster (Cricetulus griseus)

M2 group Goldfish (Gl) Goldfish (G2) C hicken M l group G oldfish Gecko C hicken Fish (Gekko gekko) (Astyanax fasciatus, (aÙ) L group

Fish (GlOl Af)

Fish (G103 Af) Gecko H um an (GCP) Fish (ROOZAf) Goldfish (R) Chicken (R) H u m an (RCP)

T able 3.5 List of the 24 vertebrate visual pigm ents w hich w ere u sed to

p roduce an am ino acid sequence alignm ent by Yokoyama (Mol Biol Evol

(1994) ll(l):32-39). The am ino acid sequences of these 24 opsins w ere com pared w ith the residues in the S group of visual pigm ents, for the production of Figure 3.8.

species. For exam ple, Thr-46, Thr-155 and Gln-314 are found only in the tw o Old W orld prim ate BCPs (hum an and talapoin m onkey; Figure 3.8). This im plies th at these substitutions occurred originally in a prim itive ancestor of the Old W orld prim ates after the divergence from the N ew W orld prim ates (marmoset) about 43 million years ago, indicated as tim e 1

in Figure 3.9 (H unt et ah, 1995). The h u m an BCP-specific resid u e Ile-50

(Figure 3.8) w ould probably have originated about 14 m illion years ago after the split of the Old W orld prim ates into the H om inoid (hum ans and

great apes) and the Cercopithecoid (talapoin monkey) branches (Time 2 in

Figure 3.9). Evolutionary analysis of other residues w hich are exclusive to m em bers of the S group of opsins (identified in blue in Figure 3.8) can be carried out in a sim ilar fashion.

Residues w hich involve non-conservative am ino acid substitutions and that are exclusive to individual m em bers of the S group of opsins m ay be involved in altering the function of the protein, such as its spectral sensitivity. An alternative interpretation is that these substitutions do not affect the norm al action of the protein. The residue Lys-22 is specific to the talapoin m onkey BCP and found in the N -term inal region of the protein (Figure 3.8). Lys-22 is the only residue carrying a positive charge at this position. The non-polar Gly-22 is present in all other m em bers of the S group and in the hu m an red and green pigm ents. The hydroxyl bearing Ser-22 is found in all vertebrate rod opsins. Arg-324 is another amino acid w hich carries a positive charge and is found to be specific to the m urine BCP, w hereas every other vertebrate opsin, including the rest of the S group of pigm ents, contain Gly-324 (Figure 3.8). W hether or not Lys-22 or Arg-324 have an effect on the norm al function of these visual pigm ents is difficult to predict w ithout experim ental evidence. Ala-180 is a m arm oset BCP-specific residue, w hereas Pro-180 is present in all the other vertebrate opsins including the other S group pigm ents (Figure 3.8). Proline residues possess a u n iq u e structure, w hich is n o t seen in any other am ino acid (Stryer, 1988). Substitution of proline for any other residue is considered a non-conserved change. Thus, Ala-180, in the m arm oset, m ay well alter the properties of the opsin (see Section 3.3.2).

3.2.42 Identification of S group-specific amino acids

Any residues indicated in blue in Figure 3.8 w hich are conserved, at the same position, in at least 7 of the 8 members of the S group are listed in Table 3.6, and are term ed 'S group-specific'. For exam ple, the residues Ala- 120 and Cys-299 were found to be exclusive to m em bers of the S group and

SUBORDER ANTHROPOIDEA

.©

T7VERA-ORDER

SUPEREAMTLY

PLATYRRHINI (New World Monkeys)

CEBOIDEA

CATARRHINI (Old World Primates)

©

CERCOPITHECOIDEA (Old World Monkeys)

HOMINOIDEA

EAMTLY CEBIDAE CALLITRICHIDAE CERCOPITHECIDAE HYLOBATIDAE HOMINIDAE PONGIDAE

COMMON NAMES

Capuchin monkeys. Howler monkeys. Squirrel monkeys

Marmosets Talapoin monkeys. Baboons,

Leaf monkeys

Gibbons Man Gorilla, Chimpanzees, Orang-utan

Figure 3.9 Classification of the primate suborder Anthropoidea (adapted from Napier, 1985).

Positions 0 and (E) refer to times within the evolution of the primates where certain species-specific residues found in their respective BCPs have arisen for a particular group (see Section 3.2.41).

Region N -term inal Helix I Position Trp-24# Gln-40 R egion Helix IV Position Leu-154 Ile-163 Val-167#

Cytoplasm ic N o n e Extracellular Val/Lys-194

Loop I Loop

n

Thr-196#

Helix II Gly-82* Ser/Thr-200

Phe-84 Cys/Asp-87 Helix V Thr-204 Ser-90 Trp/Tyr-205 Val-91 Leu-218# Val-94 Cytoplasmic N o n e

Extracellular Val-109# Loop

n

Loop I

Helix VI Gly-259

Helix III Gly-117 Ser-260

Ala-120 ♦ Tyr-265

Leu-122 Met-273#

Thr-124

Gly-125 Extracellular Asn/Tyr-277

Phe-130# Loop III His-281#

Glv-282#

Cytoplasm ic N o n e

Loop

n

Cys-299 Cvs/Ala-307

C-terminal Ala-315

T ab le 3.6 A m ino acid residues w hich are exclusive to and conserved

w ithin the S group of pigm ents (S group-specific).

A ny residues indicated in blue in Figure 3.8 w hich are conserved at the sam e p o sitio n w ith in at least 7 m em bers of the S group are term ed 'S group-specific' and are listed in the above table. In all cases w here only 7 of

the 8 S group m em bers contained the conserved S group-specific residue at

a p a rtic u la r position, it w as found th at the species w hich lacked the conserved resid u e w as eith er the chicken violet opsin or the goldfish u ltra v io le t.

♦ Gly-82 and Ala-120 are found in all the S pigm ents except the chicken violet, w hich contains residues th at are also found in 24 other non-S group opsins at these positions (Figure 3.8).

# Trp-24, Val-109, Phe-130, Val-167, Gly-195, Thr-196, Leu-218, Met-273, His-281 and Gly-282 are found in all the S pigm ents except the goldfish ultraviolet, w hich contains residues that are also found in 24 other non-S group opsins at these positions (Figure 3.8).

R esidues indicated in bold are goldfish ultraviolet opsin-specific am ino acids, w hich are also at positions containing S group-specific am ino acids identified in the Table above. For example, Ser/Thr-200 indicates that Ser- 200 is conserved in all m em bers of the S gro u p except the goldfish ultraviolet, w hich contains (Thr-200) an amino acid w hich is exclusive to a m em ber of the S group at this position (indicated in blue in Figure 3.8).

The other 24 vertebrate opsins w hich were used for com parison are listed in Table 3.5. The first 20 and last 23 amino acids from the N -term inal and C-term inal ends, respectively were not com pared due to the high degree of heterogeneity betw een these regions.

conserved in 7 and all m em bers of the S group, respectively. Thus, Ala- 120 and Cys-299 are S group specific residues, based on the above criteria (Figure 3.8 and Table 3.6).

There are no S group-specific residues situated w ith in any of the cytoplasmic loops (Table 3.6). In contrast, there are S group-specific amino acids in the extracellular loops (Table 3.6). Some of the S group-specific am ino acids w ith in the extracellular loops involve no n -co n serv ativ e substitutions com pared to the residues at the sam e site in other opsins. For exam ple, the S group-specific residue Ser-200, found in extracellular loop II, contains a hyd ro x y l group side chain, w hereas no n -ch arg ed residues Asn-200 and Val-200 are found in the R h /M l/M 2 and the L group of visual pigm ents, respectively (see Section 1.14 for definition of Rh, M l, M2 and L groups). Am ino acid changes w ithin the extracellular loops betw een gro u p s of pigm ents resulting in lo ss/g a in of a hydroxyl or a charge group m ay be involved in the spectral tuning of the visual pigm ent (see Section 3.2.53).