Bw. Morrison et al., MAT-8, A NOVEL PHOSPHOLEMMAN-LIKE PROTEIN EXPRESSED IN HUMAN BREAST-TUMORS, INDUCES A CHLORIDE CONDUCTANCE IN XENOPUS OOCYTES, The Journal of biological chemistry, 270(5), 1995, pp. 2176-2182
We recently identified a novel 8-kDa transmembrane protein, Mat-8, tha
t is expressed in a subset of murine breast tumors. We have now cloned
a cDNA encoding the human version of Mat-8 and show that it is expres
sed both in primary human breast tumors and in human breast tumor cell
lines. The extracellular and transmembrane domains of Mat-8 are homol
ogous to those of phospholemman (PLM), the major plasmalemmal substrat
e for cAMP-dependent protein kinase and protein kinase C in several di
fferent tissues, PLM, which induces chloride currents when expressed i
n Xenopus oocytes, contains consensus phosphorylation sites for both c
AMP-dependent protein kinase A and protein kinase C in its cytoplasmic
domain. In contrast, the cytoplasmic domain of Mat-8 contains no such
consensus phosphorylation sites and is, in fact, unrelated to the cyt
oplasmic domain of PLM, RNA blot analysis reveals that Mat-8 and PLM e
xhibit distinct tissue-specific patterns of expression. We show that e
xpression of Mat-8 in Xenopus oocytes induces hyperpolarization-activa
ted chloride currents similar to those induced by PLM expression, Thes
e findings suggest that Mat-8 and PLM, the products of distinct genes,
are related proteins that serve as Cl- channels or Cl- channel regula
tors but have different roles in cell and organ physiology.