Ribosome-binding protein p34 is a member of the leucine-rich-repeat-protein superfamily

Abstract

Protein p34 is a non-glycosylated membrane protein characteristic of rough microsomes and is believed to play a role in the ribosome-membrane association. In the present study we isolated cDNA encoding p34 from a rat liver cDNA library and determined its complete amino acid sequence. p34 mRNA is 3.2 kb long and encodes a polypeptide of 307 amino acids with a molecular mass of about 34.9 kDa. Primary sequence analysis, coupled with biochemical studies on the topology, suggested that p34 is a type II signal-anchor protein; it is composed of a large cytoplasmic domain, a membrane-spanning segment and a 38-amino-acid-long luminally disposed C-terminus. The cytoplasmic domain of p34 has several noteworthy structural features, including a region of 4.5 tandem repeats of 23-24 amino acids. The repeated motif shows structural similarity to the leucine-rich repeat which is found in a variety of proteins widely distributed among eukaryotic cells and which potentially functions in mediating protein-protein interactions. The cytoplasmic domain also contains a characteristic hydrophilic region with abundant charged amino acids. These structural regions may be important for the observed ribosome-binding activity of the p34 protein.