Small GTP-binding proteins associated with secretory vesicles of Paramecium.
GTP-binding proteins act as molecular switches in a variety of membrane-associated processes, including secretion. One group of GTP-binding proteins, 20-30 kDa, is related to the product of the ras proto-oncogene. In Saccharomyces cerevisiae, ras-like GTP-binding proteins regulate vesicular traffic in secretion. The ciliate protist Paramecium tetraurelia contains secretory vesicles (trichocysts) whose protein contents are released by regulated exocytosis. Using [alpha-32P]GTP and an on-blot assay for GTP-binding, we detected at least seven GTP-binding proteins of low molecular mass (22-31 kDa) in extracts of Paramecium tetraurelia. Subcellular fractions contained characteristic subsets of these seven; cilia were enriched for the smallest (22 kDa). The pattern of GTP-binding proteins was altered in two mutants defective in the formation or discharge of trichocysts. Trichocysts isolated with their surrounding membranes intact contained two minor GTP-binding proteins (23.5 and 29 kDa) and one major GTP-binding protein (23 kDa) that were absent from demembranated trichocysts. This differential localization of GTP-binding proteins suggests functional specialization of specific GTP-binding proteins in ciliary motility and exocytosis.
Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin-Madison 53706.
J B
JB
Peterson
D000818:Animals; D004591:Electrophoresis, Polyacrylamide Gel; D005089:Exocytosis; D019204:GTP-Binding Proteins / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D009154:Mutation; D010247:Paramecium / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D015800:Protozoan Proteins / Q000378:metabolism
D015800:Protozoan Proteins; D019204:GTP-Binding Proteins