Structure of nuclear ribonucleoprotein: heterogeneous nuclear RNA is complexed with a major sextet of proteins in vivo.

Abstract

Mouse erythroleukemia cells were pulse-labeled with [3H]uridine and irradiated with 254-nm light to produce covalent crosslinks between RNA and proteins in close proximity to one another in vivo. Nuclear ribonucleoprotein particles containing heterogeneous nuclear RNA were isolated and digested with nucleases, and the resulting proteins were subjected to gel electrophoresis. Proteins carrying covalently crosslinked [3H]uridine nucleotides were identified by fluorography. Heterogeneous nuclear RNA is complexed in vivo with a set of 6 major proteins having MW between 32,500 and 41,500. Analysis of chromatin fractions indicates that nascent heterogeneous nuclear RNA chains assemble with these 6 proteins as a very early post-transcriptional event. These data, and other results propose the usual order of post-transcriptional events to be: heterogeneous nuclear RNA-ribonucleoprotein particle assembly .fwdarw. poly(A) addition .fwdarw. splicing.