Cloning and sequencing of parafusin, a calcium-dependent exocytosis-related phosphoglycoprotein

A cDNA for parafusin, an evolutionarily conserved phosphoglycoprotein involved in exocytosis, has been cloned and sequenced from a unicellular eukaryote, Paramecium tetraurelia. A Paramecium cDNA library was screened with an oligonucleotide probe synthesized to an internal amino acid sequence of isolated parafusin. The insert was 3 kb long with an open reading frame of 1.75 kb. Data base searches of the deduced amino acid sequence showed that Paramecium parafusin had a 50.7% sequence identity to rabbit muscle phosphoglucomutase, although no detectable phosphoglucomutase activity has been detected in isolated parafusin. The deduced parafusin amino acid sequence had four inserts and two deletions, which might confer on the protein specific functions in signal transduction events related to exocytosis. Furthermore, searches for potential phosphorylation sites showed the presence of a protein kinase C site (KDFSFR) specific to parafusin. Southern blot analysis with probes specific for parafusin and phosphoglucomutase suggested that these proteins were products of different genes. We propose that parafusin and phosphoglucomutase are members of a superfamily that conserve homologies important for the tertiary structure of the molecules.