Transfer of proteins across membranes. I. Presence of proteolytically processed and unprocessed nascent immunoglobulin light chains on membrane-bound ribosomes of murine myeloma.

Abstract

Fractionation of MOPC 41 DL-I tumors revealed that the mRNA for the light chain of immunoglobulin is localized exclusively in membrane-bound ribosomes. It was shown that the translation product of isolated light chain mRNA in a heterologous protein-synthesizing system in vitro is larger than the authentic secreted light chain; this confirms similar results from several laboratories. The synthesis in vitro of a precursor protein of the light chain is not an artifact of translation in a beterologous system, because it was shown that detached polysomes, isolated from detergent-treated rough microsomes, not only contain nascent light chains which have already been proteolytically processed in vivo but also contain unprocessed nascent light chains. In vitro completion of these nascent light chains thus resulted in the synthesis of some chains having the same mol wt as the authentic secreted light chains, because of completion of in vivo proteolytically processed chains and of other chains which, due to the completion of unprocessed chains, have the same tool wt as the precursor of the light chain.In contrast, completion of the nascent light chains contained in rough microsomes resulted in the synthesis of only processed light chains. Taken together, these results indicate that the processing activity is present in isolated rough microsomes, that it is localized in the membrane moiety of rough microsomes, and, therefore, that it was most likely solubilized during detergent treatment used for the isolation of detached polysomes. Furthermore, these results established that processing in vivo takes place before completion of the nascent chain.The data also indicate that in vitro processing of nascent chains by rough microsomes is dependent on ribosome binding to the membrane. If the latter process is interfered with by aurintricarboxylic acid, rough microsomes also synthesize some unprocessed chains.

N-terminal processing of proteins exported by malaria parasites

“…In other eukaryotic cells, proteins destined for export enter the secretory pathway by co-translational translocation across the ER membrane mediated by the recognition of a N-terminal signal peptide and concomitant cleavage of this signal peptide by the signal peptidase complex (SPC) [16]. It has been assumed that P. falciparum proteins exported to the host RBC have their ER-type signal peptides cleaved by the parasite SPC but this type of N-terminal processing has never been definitively characterized.…”

Enhanced tumor immunogenicity through coupling cytokine expression with antigen presentation

“…Signal peptides serve as a sorting signal that targets nascent secretory proteins to sites of translocation on the ER membrane, where it is subsequently proteolytically removed from the mature chain by signal peptidase. 27,28 Signal peptides do not contain specific amino-acid residues and are variable in length (15 to as many as 50 amino acids). Kaiser et al 29 reported that many random sequences can functionally replace the secretion signal sequence of yeast invertase.…”

Mitochondrial protein import: Specific recognition and membrane translocation of preproteins

“…Thus, targeting of newly synthesized proteins to mitochondria seems to follow a general principle of intracellular protein traffic, that is the presence of signal sequences on the proteins to be transported across membranes and the existence of complementary structures on the surface of organelles which recognize the preproteins belonging to the respective cellular subcompartment (Milstein et al, 1972;Blobel & Dobberstein, 1975).…”