A 33-kDa polypeptide (termed p40), which shares an antigenic determinant with a laminin receptor and is under translational control, is believed to serve as a precursor to the receptor and to be related to the neoplastic state. The present study of subcellular localization of this protein shows it to be a cytoplasmic component not associated with the plasma membrane. Most of the cellular p40 was found to be associated with polyribosomes as well as with 40S to 60S cytoplasmic particles. Conditions that lead to polysome disruption also caused release of the polysomal form of p40 as smaller particles, and polysome reconstitution was accompanied by uptake of p40 into these structures. Because of the large abundance of this protein in the cells (six to eight copies per ribosome), it is unlikely that it represents a factor that associates with the 40S preinitiation complex. The p40-containing particles appear to represent a newly discovered structure involved in the process of polysome formation.Previous studies in this laboratory have led to the identification of an abundant mRNA species that is under translational control in a variety ofmouse tumor cells (1). The mouse mRNA codes for a polypeptide termed p40 on the basis of its mobility on polyacrylamide gels in the presence of SDS (2, 3). Analysis of a cloned cDNA for the p40 mRNA species showed it to code for a 33-kDa polypeptide (4). A human cDNA clone isolated from a colon carcinoma on the basis of the unusually high abundance of mRNA in the tumor (5) showed nearly complete homology to the mouse p40. A possible relationship between p40 and a laminin-binding protein was suggested by the fact that it contains an octapeptide sequence identical to that of a peptide derived from the 68-to 72-kDa laminin receptor (6). Moreover, a mouse cDNA clone obtained by screening with an antibody to the laminin receptor (7) proved to be essentially identical to the p40 cDNA. A relationship between p40 and the neoplastic state has been proposed (7).The present study deals with characterization of p40 in the cell. We found no evidence for association ofthis protein with membranes in cell extracts. Major portions of p40 are associated with 40S to 60S cytoplasmic particles and with polyribosomes in various cell types. Shifts of p40 between the small particle state and polysomes were observed under conditions that lead to generation or loss of polysomes both in vivo and in cell extracts. We conclude from our results that p40 is part of a cytoplasmic structure that is involved in the translation process.
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