(36), but VP1 is essential for formation of infectious AAV type 2 (AAV-2) particles (17, 42, 50). VP2 cotransports VP3 into the nucleus before capsid assembly (18,36). VP3 alone also forms capsids but only when targeted to the nucleus (18). Encapsidation of the AAV-2 genome likely occurs in the nucleoplasm in areas where capsids, Rep proteins, and DNA colocalize (52). Detailed analysis of the protein-protein interactions of Rep and VP proteins favors a model by which Rep-tagged DNA initiates packaging by interaction with capsid proteins (11). Several of the above-mentioned studies of the AAV-2 capsid assembly process were aided by using monoclonal antibodies (MAbs) directed against the capsid proteins.AAV-2 infects a broad range of cells by binding to its primary receptor, heparan sulfate proteoglycan (47). Two types of coreceptors, ␣ v ␤5 integrin and fibroblast growth factor receptor

Using monoclonal antibodies we have localized a polypeptide, appearing on gel electrophoresis with a Mr of approximately 38,000 and a pI of approximately 5.6, to the granular component of the nucleoli of Xenopus laevis oocytes and a broad range of cells from various species. The protein (NO38) also occurs in certain distinct nucleoplasmic particles but is not detected in ribosomes and other cytoplasmic components. During mitosis NO38‐containing material dissociates from the nucleolar organizer region and distributes over the chromosomal surfaces and the perichromosomal cytoplasm; in telophase it re‐populates the forming nucleoli. With these antibodies we have isolated from a X. laevis ovary lambda gt11 expression library a cDNA clone encoding a polypeptide which, on one‐ and two‐dimensional gel electrophoresis, co‐migrates with authentic NO38. The amino acid sequence deduced from this clone defines a polypeptide of 299 amino acids of mol. wt 33,531 which is characterized by the presence of two domains exceptionally rich in aspartic and glutamic acid, one of them flanked by two putative karyophilic signal heptapeptides. Comparison with other protein sequences shows that NO38 is closely related to the histone‐binding, karyophilic protein nucleoplasmin: the first 124 amino acids have 58 amino acid positions in common. Protein NO38 also shows striking homologies to the phosphopeptide region of rat nucleolar protein B23 and the carboxyterminal region of human B23. We propose that protein NO38, which forms distinct homo‐oligomers of approximately 7S and Mr of approximately 230,000, is a member of a family of karyophilic proteins, the ‘nucleoplasmin family’. It is characterized by its specific association with the nucleolus and might be involved in nuclear accumulation, nucleolar storage and pre‐rRNA assembly of ribosomal proteins in a manner similar to that discussed for the role of nucleoplasmin in histone storage and chromatin assembly.

Abstract, The formation of daughter nuclei and the reformation of nucleolar structures was studied after rnicroinjection of antibodies to RNA polymerase I into dividing cultured cells (PtK2). The fate of several nucleolar proteins representing the three main structural subcomponents of the nucleolus was examined by immunofluorescence and electron microscopy. The results show that the RNA polymerase I antibodies do not interfere with normal mitotic progression or the early steps of nucleologenesis, i.e., the aggregation of nucleolar material into prenucleolar bodies. However, they inhibit the telophasic coalescence of the prenucleolar bodies into the chromosomal nucleolar organizer regions, thus preventing the formation of new nucleoli. These prenucleolar bodies show a fibrillar organization that also compositionaUy resembles the dense fibrillar component of interphase nucleoli. We conclude that during normal nucleologenesis the dense fibrillar component forms from preformed entities around nucleolar organizer regions, and that this association seems to be dependent on the presence of an active form of RNA polymerase I.