GRITERÍO: DESARROLLO DE LAS ACTIVIDADES

A nalysis o f the tyrosine phosphorylation o f specific proteins associated w ith cytoskeletal

d e v e lo p m en t sh o w ed that C sk reg u lated-S rc w as resp o n sib le for p h o sp h o ry la tio n o f

co rta c tin , ten sin , p ax illin and Fak, w hile Fyn acted on ly on pax illin and Fak, C sk

expression w as not noted as being w idespread through all tissues. This w ould suggest that

only certain Src fam ily m em bers require a high level o f precise regulation (supplied by

C sk) to e ffe c t th e ir c o rre c t fu n ctio n , such as F yn a n d L ck in T C R sig n a llin g .

A lternatively, oth er related S rc-regulating tyrosine kinases exist w hich have yet to be

cloned. In support o f the latter argum ent, a hom ologue o f the csk gene w as isolated by M cV icar et al. (1994) called Isk (expressing ).This is expressed in hum an brain, resting and activated N K cells, and activated T-cells. A lso, K lages et at. (1994) reported the iso latio n o f a b rain -sp e c ific Src kinase gene in m ice c a lled ctk w h ic h is 53% h o m o lo g o u s w ith m urine c s k and encodes a 5 2 kD a p ro tein p roduct. O th e r C sk -lik e kinases include M A T K , the hum an hom ologue o f ctk, the hum an h ae m o p o ietica lly restricted FIYL gene, and a m urine brain and T cell-sp ecific gene ntk. (B ennett et at.,

1994; Sakano et a i , 1994; C how et al., 1994). E vidently there is now a grow ing fam ily o f C sk proteins w hich act in a tissue and/or target protein-specific m anner.

In c o n tra st to the p h o sp h o ry la tio n o f Src p ro te in s, little is know n c o n c e rn in g the

dep h o sp h o ry latio n . The best know n exam ple o f specific p ro tein tyrosine p h o sphatase

(PT Pase) activity is the leukocyte com m on antigen c o -recep to r C D 45. A partial am ino

acid sequence (PTP IB ) w as isolated by Tonks et al. (1988) w hich had no hom ology to know n serine/threonine (S/T) phosphatases. PTP IB w as then found to be hom ologous to

C D 45, w hich at that tim e had no know n function. A lthough an intrinsic PT P ase dom ain

had p rev iously been n oted in functional assays, it becam e c le a r that cro ss-lin k in g o f

C D 45 to C D 4/C D 8 by M H C class II interaction (see section 1.2.1.4) physically brings

C D 4/C D 8-associated Lck into proxim ity with the PTPase dom ains o f CD 45. T his allow s

d ep hosphorylation o f Lck at Y -505, w hich in turn activates L ck and allow s interaction

B urgess et al., 1992). R ecently, it was show n that C sk can also p h o sp horylate C D 45, activating its PT P ase activity and generating a phosphotyrosine residue that can bind to

the SH 2 dom ain o f L ck (A utero et al., 1994). This suggests that the regulatory circu it involving Csk, C D 45 and L ck is m ore com plicated than w as previously thought.

T he m echanism o f phosphorylation and deph o sp h o ry latio n o f Y -416 rem ains unclear.

C learly it w ould be counter-productive if Csk were to phosphorylate this tyrosine, thereby

favouring an active conform ational state (see section 1.2.1.2). Likew ise, PTPase m ediated

dephosphorylation o f Y -416 w ould favour an inactive conform ation.

1.2.1.3 Expression and function o f Src fam ily proteins

T he pattern o f src gene fam ily expression can be divided in tw o groups: those that are u b iq u ito u sly ex p ressed (c-5 rc. c-y e s , y r k and f y n ) and those ex p re sse d so lely in the h aem o p o ietic cell lineages {c-fgr, hlk, lck, h c k and ly n ). In a num ber o f stu d ies, the targeted disruption o f specific src fam ily genes in transgenic m ice has been em ployed to help elucidate their function. A sum m ary o f these is show n in Table 1.2.2.

Ubiquitously expressed Src fa m ily proteins

A lthough Src is expressed in m ost cell types, attention has focused on a few cell types

w here expression is particularly high. Src is expressed at a high level in platelets, w here it

is lo calised to the p lasm a m em brane and the su rface c o n n e cte d c a n a lic u la r sy stem ,

a lth o u g h its fun ctio n is u n c le a r (F errell et a i , 1990). T he c - s r c gene is seen to be e x p ressed in m any tissues but is particularly abundant in the developing chick em b ry o

K

and in the adult central nervous system (CN S). Src is lo calised to nerve grow th cones

w here it has been su ggested that it is involved in reg u latin g neurite e x ten sio n during

contact-m ediated guidance through neural developm ent tow ards its target (M aness et al.,

1988). Src m ediated phosphorylation o f a- and (3- tubulin, proteins associated w ith grow th