Intranuclear appearance of the phosphorylated form of cytoskeleton-associated 350-kDa proteins in U1-ribonucleoprotein regions after growth stimulation of fibroblasts.
Cytoskeleton-associated 350-kDa and 80-kDa polypeptides, which were immunoprecipitated with polyclonal antibody against microtubule-associated protein 1 (MAP-1), were rapidly phosphorylated on mitogenic stimulation of quiescent fibroblasts with serum or growth factors. The enhanced phosphorylation was evident within 5 min and reached a maximum 2 hr after the stimulation. Phosphorylated MAP-1 analogues were first detected in the cytoplasm around the microtubule-organizing center and then in the nucleus by immunofluorescent staining with a monoclonal antibody that recognized the phosphorylated form of MAP-1. The monoclonal antibody reacted with the 350-kDa protein in immunoblot analysis and immunostained intranuclear speckles; both immunoreactions were abolished by treatment with alkaline or acid phosphatase. The nuclear speckles stained by the monoclonal antibody were also stained by anti-U1 small nuclear ribonucleoprotein antibodies on double immunofluorescence, suggesting that the stained regions are sites of maturation of messenger RNA. These results support the idea that part of the cytoskeleton-associated 350-kDa protein is phosphorylated and transferred to the nuclear region of mRNA modification as a common early process after growth stimulation.
D000818:Animals; D000911:Antibodies, Monoclonal; D000918:Antibody Specificity; D002451:Cell Compartmentation; D002453:Cell Cycle; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism / Q000648:ultrastructure; D005786:Gene Expression Regulation; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D008970:Molecular Weight; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011518:Proto-Oncogene Proteins / Q000235:genetics; D012323:RNA Processing, Post-Transcriptional; D051381:Rats; D012261:Ribonucleoproteins / Q000378:metabolism*
D000911:Antibodies, Monoclonal; D008869:Microtubule-Associated Proteins; D010750:Phosphoproteins; D011518:Proto-Oncogene Proteins; D012261:Ribonucleoproteins