From structure to function: possible biological roles of a new widespread protein family binding hydrophobic ligands and displaying a nucleotide binding site.

A cytosolic 21-23 kDa protein isolated from bovine brain was demonstrated to bind hydrophobic ligands, particularly phosphatidylethanolamine. The protein was encountered in numerous tissues of several species. High expression of the mRNA encoding the 21-23 kDa protein was found in rat testes. Immunohistochemical studies showed the presence of the 21-23 kDa protein in the elongated spermatids and epididymal fluid of rat testis and in brain oligodendrocytes of developing rats. As the bovine, human and rat brain 21-23 kDa proteins had only few sequence homologies with already know proteins, ti was concluded that they belong to a new protein family. In order to get additional information on the structural features of the 21-23 kDa protein, we built a molecular model which displayed a nucleotide binding site. The affinity of the bovine brain 21-23 kDa protein towards nucleotides as well as its association with cytosolic proteins and small GTP-binding proteins were demonstrated. Recently, significant sequence homologies were found with an antigen from Onchocerca volvulus, a fruit fly odorant-binding protein and the yeast protein TFS1 which is a dosage-dependent suppressor of CDC25 mutations. A positive regulation of RAS is carried out by CDC25 product which facilitates the GDP/GTP exchange on RAS proteins. These results imply that 21-23 kDa proteins function in oxidoreduction reactions and signal mechanisms during cell growth and maturation.