Chemistry Reference
In-Depth Information
11.6
Cell - Surface (Transmembrane) PG s
11.6.1
Syndecans
[16 - 22]
The syndecan (SDC) family of HSPGs comprises four members which are
expressed by different genes on four different chromosomes in mammals (see
Table 11.1). Their expression is strongly regulated in a tissue-specifi c and devel-
opmentally-dependent manner. SDC1 is most abundant in endothelial cells and
epithelial cells, SDC2 (fi broglycan) is the predominant syndecan in fi broblasts and
smooth muscle cells, SDC3 (
N
-syndecan) is highly expressed in the central nervous
system, and SDC4 (amphiglycan, ryudocan) is more ubiquitously expressed by
multiple cell types. In mammals SDC1 and 3 and SDC2 and 4 share stronger
sequence homologies, and therefore represent two subfamilies.
11.6.1.1
Structure
The genes of the syndecans are divided into fi ve exons which code for three dif-
ferent domains (Figure 11.6). (i) The extracellular domain contains GAG attach-
ment sites (between 3 and 8) for HS (N-terminal) and CS chains (in proximity to
the plasma membrane). Various specifi c functions of the syndecans depend on
the presence of sugar chains. (ii) The transmembrane domain is highly conserved
among the syndecans (24 amino acid residues) and promotes dimerization. (iii)
The cytoplasmic domain of syndecans can be divided into three regions - two
constant regions (C1 and C2) that are highly conserved among the syndecans
separated by a variable (V) region that is specifi c for each family member. The last
four amino acids of the cytoplasmic domain (EFYA) are identical in all syndecans
and represent the binding sequence for PDZ (PSD-95/Disk large/ZO-1)-contain-
ing proteins. The cytoplasmic part also contains four tyrosine residues which may
serve as putative phosphorylation sites: two tyrosines are located in C1 and each
one in the V and C2 sections, respectively.
Figure 11.6
Domain structure of syndecan pro-
tein core. TM: transmembrane domain; C1 and
C2: constant regions of the cytosolic domain;
V: variable region of the cytosolic domain.
