Subcellular localization of protein phosphatase type 1 isotypes in mouse osteoblastic cells

The cytolocalization of protein phosphatase type 1 catalytic subunits in exponentially growing mouse osteoblastic MC3T3-E1 cells was determined. Formaldehyde-fixed and alcohol-permeabilized cultured cells were reacted with the PP1 alpha, PP1 delta, PP1 gamma 1, and PP1 gamma 2 antibodies using immunohistochemical methods. With PP1 alpha antibody intense staining occurred in the nuclei, while with PP1 delta antibody nucleolus-like bodies were intensely stained. PP1 gamma 1 localized in the perinuclear region and in the nucleus of the cultured cells, with the staining reaction of the former being much stronger than that in the latter. An immunoreaction did not occur in the cells interacted with PP1 gamma 2 antibody or with the normal rabbit serum. Proteins were prepared from the exponentially growing cells and subconfluent cells. Cellular fractionation was also done with the exponentially growing cells and proteins were prepared from each fraction. Each protein preparation was subjected to SDS-PAGE followed by Western blot analysis with the antibodies. PP1 alpha recognized the 38 kDa proteins mainly present in the nucleus, whereas PP1 delta interacted with the proteins in the nucleolar fraction whose molecular weight was estimated as 37 kDa. PP1 gamma 1 antibody recognized a band corresponding to an estimated molecular weight of 36 kDa mainly in the cytosolic fraction. PP1 gamma 2 antibody and the normal rabbit serum did not interact with any proteins prepared from the cultured cells. Our observations show that four different isozymes of protein phosphatases occupy distinct compartments in MC3T3-E1 cells. This differential distribution suggests that these isozymes may play different roles in cellular functions.