Human cyclin B1 is targeted to the nucleus in G1 phase prior to its accumulation in the cytoplasm.

Abstract

The immunolocalisation patterns of cyclin B1 have been investigated in various tumour-derived and untransformed human cells, using one polyclonal (B7/B8) and two different monoclonal anti-human cyclin B1 antibodies, GNS1 and GNS11. In actively dividing cell populations, GNS11 reveals uniquely a cytoplasmic pool of cyclin B1 that rapidly increases after the onset of S phase; yet, B7/B8 and GNS1 detect, besides this cytoplasmic cyclin B1 population, a moderate but clear nuclear concentration of the protein in cells that have not yet entered S phase. As for confluent populations of untransformed and tumour cells, they become enriched in G1- and G2-arrested cells that characteristically display a discrete nuclear or an intense cytoplasmic GNS1 immunostaining respectively. Altogether, our immunofluorescence data conjugated with the results of a detailed biochemical analysis suggest that human cyclin B1 would exist in situ as two distinguishable molecular entities differentially susceptible to in vitro degradation and exhibiting different timing, kinetics and site of accumulation during the cell cycle: one form (recognized by the polyclonal and GNS1 antibodies but apparently not by GNS 11) starts to build up inside the nucleus prior to entry in S phase and the other (in which both the GNS11 and GNS1 epitopes are readily accessible) emerges and accumulates in the cytoplasm beyond the G1/S boundary.