Immunologically-related nucleic acid-binding proteins associated with the nuclear matrix of Physarum polycephalum

The nuclear matrix of Physarum polycephalum is composed of two abundant polypeptides of 27 and 38 kDa as well as numerous minor polypeptides of various molecular weight. By contrast, the nuclear matrix of vertebrates consists of three major (the lamins) and many minor polypeptides mainly in the 60-70 kDa molecular weight range. In order to better characterize the major nuclear matrix proteins of P. polycephalum and, perhaps, define their relationship with the major nuclear matrix proteins of vertebrates, we have purified the abundant nuclear matrix proteins of P. polycephalum. In Western blot analyses, polyclonal antibodies raised against the purified 27 kDa polypeptide recognised polypeptides of 50 kDa, 45 kDa and several low molecular weight species (14-21 kDa) in the P. polycephalum nuclear matrix. The polyclonal antibodies did not react with the other abundant nuclear matrix protein of 38 kDa from P. polycephalum nor with polypeptides of the mouse nuclear matrix. Two-dimensional gel electrophoresis showed that the major nuclear matrix proteins of P. polycephalum were more basic than the major nuclear matrix proteins of vertebrates, the lamins. Moreover, both the 27 and 38 kDa polypeptides are post-translationally modified by either d-mannosyl or d-glycosyl moieties, and not by phosphorylation as has been demonstrated for the vertebrate lamins. DNA-binding assays further revealed that the immunologically related polypeptides of 50 kDa, 45 kDa, 27 kDa and low molecular weight species of 14-21 kDa preferentially bound single-stranded DNA, but the 38 kDa polypeptide of Physarum matrix did not. Based on these findings, we conclude that the abundant nuclear matrix protein of 27 kDa belongs to a group of immunologically-related nucleic acid-binding proteins, and is immunologically and functionally distinct from the other major nuclear matrix protein of 38 kDa from P. polycephalum and the vertebrate lamins.