The size distributions of proteins, mRNA, and nuclear RNA.

Abstract

The frequency distributions of size (molecular weight) and of numbers of subunits were determined from lists of over 500 mammalian and bacterial proteins. The size distribution of polypeptides is well fitted by a lognormal distribution with a median value of about 40,000 daltons and a deviation of 1.8. About 60% of all proteins exist in multimeric aggregates. Of the multimers 75% have either two or four subunits while less than 1% have an odd number of subunits that is greater than three. Over 90% of the time, a given multimer is composed of subunits of nearly equal size so that the size of a N-mer is lognormally distributed with a median value of N x 40,000 daltons and a deviation of 1.8. The distribution of polypeptide size and subunit number is similar for mammalian and bacterial proteins as well as for intracellular and extracellular proteins. The sedimentation profiles of mRNA from HeLa and CHO cells indicate that the lengths of mammalian mRNA are lognormally distributed with a median value of 1.4 kb and a deviation of 2.0. This implies that, on the average, a mRNA species is only about 25% larger than the mature polypeptide it codes for. Therefore, at most a small fraction of mammalian mRNA could code for large precursor polypeptides which are then cleaved into a number of mature polypeptides (like polio mRNA), or for 3′ coterminal mRNAs where the larger species contain the information for up to four proteins (like adenovirus mRNA). The sedimentation profile of nascent nuclear RNA from HeLa suggests that the length distribution of transcription units has 2 components: An exponential component that decays with a half-length of 10–15 kb, and a high frequency of very short molecules. However, other distributions (for example, the lognormal distribution) of transcription unit lengths could also be consistent with the data if one or more of the following occurred: Physiological cleavage of nascent chains, perturbation of non-rRNA transcription by actinomycin D, or degradation during isolation. The length distribution of HeLa nuclear RNA labeled for 60 min is similar to that of nascent nuclear RNA, indicating that a completed hnRNA chain is quickly transported or degraded after being cleaved.