H. Fujita et al., ENDOGENOUS SYNTAXIN-2, SYNTAXIN-3 AND SYNTAXIN-4 EXHIBIT DISTINCT BUTOVERLAPPING PATTERNS OF EXPRESSION AT THE HEPATOCYTE PLASMA-MEMBRANE, Biochemical journal, 329, 1998, pp. 527-538
To investigate the mechanisms regulating polarized vesicle delivery to
the cell surface in hepatocytes, we have characterized the endogenous
plasma membrane (PM)-associated syntaxins. These integral membrane pr
oteins are components of the membrane docking/fusion apparatus and are
thought to function as vesicle receptors at the PM. In hepatocytes, t
he PM is divided into two domains, the apical and basolateral. If synt
axins are mediating the specific recognition of vesicles delivered to
either membrane surface, the simple prediction is that each domain exp
resses one syntaxin isoform. However, we report that rat hepatocytes e
xpress three endogenous PM-associated syntaxin isoforms, syntaxins 2,
3 and 4. By biochemical subfractionation, we determined that the synta
xins exhibit distinct, but overlapping patterns-of expression among th
e PM domains. Syntaxin 4 is primarily expressed at the basolateral sur
face while syntaxins 2 and 3 are enriched at the apical PM. The immuno
localization of syntaxins 2 and 4 in rat hepatocytes and PM sheets rev
ealed similarly complex patterns of PM expression with enhanced apical
staining for both. A significant proportion of syntaxin 3 (25 %) was
detected in subcellular fractions containing transport vesicles. We ha
ve used quantitative immunoblotting to determine that the syntaxins ar
e relatively abundant PM molecules (11-260 nM) in rat liver, spleen an
d kidney. Also, we determined that the syntaxin binding protein, Munc-
18, is present at concentrations from 1.5-20 nM in the same tissues. A
lthough this fundamental quantitative and morphological information is
lacking in other systems, it is critical not only for defining syntax
in function, but also for predicting the specific mechanisms that regu
late vesicle targeting in hepatocytes and other tissues.