The large high mobility group proteins of rainbow trout are localized predominantly in the nucleus and nucleoli of a cultured trout cell line.

On the basis of immunofluorescence and biochemical studies, it has been suggested that the concentration of nonhistone chromosomal protein high mobility group 1 may be higher in the cytoplasm than in the nucleus of mammalian cells (Bustin, M., and Neihart, N. K. (1979) Cell 16, 181-189). In view of the possible implications of this finding, we have examined the in situ location of trout proteins HMG-T1 and HMG-T2, which are analogous to the mammalian proteins HMG-1 and HMG-2. Antibodies prepared against purified HMG-T2 were shown to react only with HMG-T1 and HMG-T2, but not with any other chromosomal proteins from trout. This has been established using a modified immunoautoradiographic techique involving CNBr-activated paper transfers of proteins separated on regular sodium dodecyl sulfate-polyacrylamide gels. Using the indirect immunofluorescence technique to examine the subcellular location of HMG-T (T1 and T2) proteins in a cultured cell line of rainbow trout, we find that these proteins are located primarily in the nucleus of these cells. The fluorescence in the nucleolar regions is even more intense than in the nonnucleolar regions. The cytoplasmic regions show only a weak fluorescence which may be due to low levels of HMG-T proteins in the cytoplasm, since preincubation of anti-HMG-T2 with purified HMG-T2 abolishes the nuclear as well as cytoplasmic fluorescence.