The biosynthesis of protamine in trout testis. I. Intracellular site of synthesis

Abstract

At a late stage of spermatogenesis, protamine, a sperm-specific protein of low molecular weight and high arginine content ([Formula: see text] of the total residues), appears in the nuclei of testis cells in salmonid and, related fish. This newly synthesized small protein progressively replaces the somatic histones in combination with DNA.Two column chromatography techniques (Bio-Gel P-10 and CM-cellulose) which are capable of completely separating protamine from histones and other testis basic proteins have been employed to identify and characterize newly synthesized protamine. When protamine synthesis begins, histone synthesis is decreased and eventually ceases. By pulse labelling testis cell suspensions for different lengths of time and analyzing the amount of 14C-protamme found in the cytoplasm and in the nucleus, the site of protamine synthesis can be shown to be in the cytoplasm. Further, a cell-free, isolated cytoplasmic fraction can incorporate 14C-arginine into whole protamine molecules, while both an isolated nuclear fraction and high-speed supernatant were relatively inactive. Sedimentation analysis of pulse-labelled testis ribosomes indicates that protamine is synthesized on a class of small polysomes, the disomes. Because of its highly basic nature, the release of protamine from the ribosomes and its subsequent transport into the nucleus, which appears to be rapid, may require special mechanisms.