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SIOOB is primarily expressed in the brain, and has been shown to bind 24

proteins in a Ca^'^-dependent fashion (reviewed by (Zim m er et al., 1995) and

(Donato, 1999)). In many situations SIOOB is proposed to block phosphorylation

sites. This SlOO protein inhibits assembly of type III intermediate filaments and

m icrotubules, and stimulates the actom yosin ATPase by interacting with the

inhibitory proteins, caldesmon and calponin. Therefore, SIOOB may be a major

regulator of the cytoskeleton. SIOOB also regulates energy metabolism by activating

and inhibiting enzymes. SIOOB modulates cell signalling by interacting with kinases

Chapter 1: Introduction

Extracellular homodimeric SIOOB promotes neurite extension, via induction

of an increase in [Ca^"^]; (Barger et al., 1992). This neurotropic effect only occurs at

low concentrations, as higher concentrations of the homodimer can induce apoptosis

(Fulle et al., 1997). SIOOB can also stimulate the expression of (3-amyloid precursor

protein in neuronal cultures (Li et al., 1998). Therefore, the elevated level of SIOOB

in D ow n’s syndrome and Alzheimer’s disease may contribute to the neuropathology

of these diseases. Currently, no receptor for SIOOB has been identified.

1.2.d.iii.2 SlOOAl

The target proteins of SIOOB and SlOOAl are very similar, which is reflected

in their high homology of 60% (see (Donato, 1999)). The only known SlOOAl

specific role is the direct regulation of Ca^^ homeostasis by increasing the affinity of

the Ca^^ release channel, ryanodine receptor, for its ligand (Treves et al., 1997).

SlOOAl has not been ascribed an extracellular function or been associated with any

pathologies.

1.2.d.iii.3 SI 00 A4

S I 00A4 is highly expressed in the cells of kidneys, salivary glands, stomach

and intestine, that are involved in ion transport. S I 00A4 has also been detected in

neurons, smooth muscle cells and some leukocytes. The expression and putative

functions have recently been reviewed in (Barraclough, 1998). S I 00A4 was first

identified because it is associated with increased proliferation, m orphological

changes and metastatic potential in cultured cells. This SlOO protein has also been

linked to metastasis/malignancy in human tumour biopsies. Transfection of benign

cell lines with S I 00A4 induced metastatic activity in vivo (Davies et al., 1993), and

sim ilar results were obtained with a transgenic model (Davies et al., 1996).

U nfortunately the m echanism by which S I 00A4 induces a m etastatic

phenotype is unknown, but an increase in motility and altered morphology has been

proposed. S I 00A4 has beenlshownto interact with F-actin, tropomyosin and myosin.

The interaction with myosin prevents phosphorylation by PKC, and also inhibits its

ATPase activity (see (Barraclough, 1998)).

1.2.d.iii.4 S100A6

S100A6 is primarily expressed in fibroblasts and epithelial cells (Kuznicki et

al., 1992), and is upregulated by growth factors (Calabretta et al., 1986). S100A6 is

also over-expressed in many tumours (Ilg et al., 1996a). Again the function of

S100A6 has not been defined, but some characteristics have been elucidated. Ca^*

induces S100A6 to bind cytoskeletal associated proteins, such as tropom yosin

(Golitsina et al., 1996) and caldesmon (Mani and Kay, 1995), annexins XI-A (Sudo

and Hidaka, 1999), II and VI (Zeng et al., 1993), as well as glyceraldehyde-3-

phosphate dehydrogenase (Zeng et al., 1993). Interestingly, S100A6 also associates

with plasma and nuclear membranes in a Ca^^-dependent manner, but is localised to

the cytoplasm during cell division (Stradal and Gimona, 1999).

1.2.d.iii.5 SIOOAIO

SIOOAIO, the only SlOO protein that does not bind Ca^^ interacts with and

inhibits cytosolic phospholipase Aj (PLAj) (Wu et al., 1997). However, SIOOAIO is

most well characterised as a regulator of annexin II, with which it form s a

heterotetrameric complex (annexin II2-SIOOAIO2). Annexin II2-SIOOAIO2 binds to

glial fibrillary acidic protein (GFAP), and stimulates assem bly of the type III

intermediate filaments. Interestingly, this appears to oppose the effect of SIOOB,

which binds to GFAP at a different site (Bianchi et al., 1995; Garbuglia et al., 1995).

The heterotetramer also shows Ca^'^-dependent binding to and bundling of F-actin

Chapter 1: Introduction

Ca^^-triggered exocytosis by adrenal chromaffin cells (reviewed by (Burgoyne et al.,

1991)). Specifically, annexin II2-SIOOAIO2 contributes to Ca^^-induced membrane

localisation of vesicles as well as Ca^^-triggered granule fusion and aggregation.

There is also evidence that the heterotetram er can link F-actin to the mem brane

(reviewed in (Gerke and Moss, 1997)). Thus, it has been hypothesised that this

annexin/SlOO complex is involved in linking the cytoskeleton with cell membranes,

vesicles and other cellular components.

1.2.d.iii.6 SlOOAl 1

SlOOAl 1 binds to F-actin (Sakaguchi et al., 2000), and inhibits actin-

activated myosin Mg^^-ATPase (Zhao et al., 2000) in a Ca^“^-dependent manner.

Interestingly, in fibroblasts SlOOAl 1 is predom inantly associated with actin

filaments of leading edge pseudopodia, and overexpression of the protein leads to

increased pseudopodia formation (Sakaguchi et al., 2000). SlOOAl 1 associates with

annexin I, and the heterotetram er (annexin I2-SIOOAIF) may contribute to the

formation of the cornified envelope in epiderm al kératinocytes (review ed by

(D onato, 1999)). Upon confluency, cytoplasm ic SlOOAl 1 in fibroblasts is

phosphorylated, which results in translocation to the nucleus. This has been

associated with contact inhibition, and does not occur in im m ortalised cells

(Sakaguchi et al., 2000).

1.3 The MRP Proteins

Migration inhibitory factor-related protein (MRP) - 8 and -1 4 are two highly

homologous SlOO proteins designated S100A8 and S100A9, respectively (Schafer et

al., 1995). M RP- 8 is a 10.8kDa protein, and has also been named p8, L I light chain,

calgranulin A and cystic fibrosis antigen (CFA). MRP-14 is the largest known SlOO

includes p l4 , L I heavy chain and calgranulin B. The M RP proteins are known to

form a heterodimer (Edgeworth et al., 1991), which has been named calprotectin.

The 8 and 14 refers to the molecular weights of the molecules as determined by SDS

PAGE.