Prosomes,subcomplexes of untranslated mRNP

PROSOMES are a novel class of small RNP particles of uniform morphology, but of variable RNA (pRNA) and protein composition (about 650 000 MW; 12 nm diameter in the EM). They were discovered as subcomplexes of free mRNP, tightly attached to inactive mRNA in the cytoplasm. The pRNAs hybridize stably to mRNA. Prosomes associate in vitro to mRNA and inhibit cell free protein synthesis inducing an mRNA structure unable to interact with ribosomes. Many types of prosomes were observed. The individual particle is made up by a variable combination of about 20 characteristic proteins and one or several pRNA. Some prosomal proteins are glycosylated, phosphorylated and, possibly, ADP-ribosylated and are highly conserved in evolution whilst others vary with the species and the mRNA population they are associated to. A protease activity was found associated to prosomes.The function(s) of the prosomes is(are) still unknown. The differential inhibition of in vitro protein synthesis points to a capacity to recognize mRNA and to keep it in an inactive state. The observation with the aid of monoclonal antibodies (pMABs) that prosomes and thus mRNP are attached to the intermediate filaments (IF) raises the question if one of the functions of the IF might be in the topological distribution of mRNA within the cell. Similar to the cytokeratin fibers, the prosome networks bridge neighboring cells at specific positions. — The nucleus also contains some prosomal antigens, located on chromosomes and on the nuclear matrix. Their presence and distribution in the cell compartments varies with the cell type and the prosomal antigen probed.Oocytes contain large amounts of prosomes. In embryonic development, the synthesis of individual prosomal proteins starts progressively after the blastula stage and resumes fully in gastrulation only; cleavage and blastula stage prosomes are thus of maternal origin. The nucleo-cytoplasmic distribution of prosomal antigens changes in embryos, with the stage of development and type of differentiation. In human tissues specific patterns of prosomal antigens were found in function of cell type and differentiation.In view of these data, the hypothesis may be formulated that prosomes are a population of mRNA-linked RNP which includes particles of varying individual composition and hence specificity. Attached to IF sub-networks, specific types of prosomes might accompany families of mRNA in function of the physiological state and the specialisation of given differentiated cell types. The cell-type specific organisation of the IF networks might be related to the messenger RNA complement of a given cell, and to its status of gene expression. The prosomes might thus have a function in controlling the transport, distribution and control of activity of specific mRNAs in the cell.