In this study, DNA-depleted nuclear protein matrices are isolated from HeLa S3 cells. These nuclear matrices consist of peripheral laminae, residual nucleoli, and internal fibrillar structures . High molecular weight, heterogeneous nuclear RNA (hnRNA) is quantitatively associated with these structures and can be released intact only by affecting the integrity of the matrices . It is, therefore, concluded that hnRNA is part of a highly organized nuclear structure.By irradiation of intact cells or isolated nuclear matrices with ultraviolet light, proteins tightly associated with hnRNA can be induced to cross-link with the RNA . Performing the crosslinking in vivo is an extra guarantee that only hnRNA-protein (hnRNP) complexes existing in the intact cell are covalently linked . Such hnRNP complexes were isolated and purified under conditions that completely dissociate nonspecific RNA-protein complexes .By comparison of the hnRNP found in nuclear matrices and the published data on the composition of hnRNP particles, it was found that the so-called hnRNP "packaging" proteins (32,000-38,000 mol wt) were not efficiently cross-linked to hnRNA by UV irradiation . They were, however, present in the matrix preparations, bound to hnRNA, because they were released from nuclear matrices after ribonuclease treatment of these structures . On the other hand, two major hnRNPs (41,500 and 43,000 mol wt) were efficiently cross-linked to hnRNA . These proteins were not released by ribonuclease treatment, which suggests that they are involved in the binding of hnRNA to the nuclear matrix .Althoughin the last decademuch knowledge has been acquired about hnRNA at the molecular level, little is known about the role ofvarious cellular structures and components in transcription, RNA processing, and RNA transport to the cytoplasm. To investigate such a role, studies have been undertaken to identify protein and RNA molecules that interact with heterogeneous nuclear RNA (hnRNA) . hnRNA can be extracted from nuclei in the form of hnRNA-protein (hnRNP) particles (reviewed by Heinrich et al. [1] and Van Venrooij and Janssen [2]) that have a repeating subunit structure composed of 200-300 A spherical particles connected by a ribonuclease-sensitive strand (3, 4). Small nuclear RNA seem to be integral parts of hnRNP particles (5-8). The protein composition of hnRNP particles is very complex and seems to be related to the isolation procedure used. Although nonspecific binding of proteins to the RNA during the isolation of the particles has never rigorously been excluded,most workers (9-11) agree on the presence of some predominant proteins in the 30,000-44,000 mol wt range.The isolation of hnRNP particles mostly involves nuclear breakage (for example, by sonication) or extraction of the nuclei with buffer solutions for prolonged periods at a relatively high temperature . In general, the amount of hnRNP released from the nuclei seems to depend very much on the degree of nuclear disintegration achieved (2) .Detergent-treated nuclei from euka...

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