KARYOPHOBIC PROTEINS - A CATEGORY OF ABUNDANT SOLUBLE-PROTEINS WHICH ACCUMULATE IN THE CYTOPLASM

The cytoplasm of oocytes of Xenopus laevis is enriched in several soluble proteins which are either absent from the nucleus or are present there at very low concentrations. These molecules, collectively referred to as karyophobic (from the Greek verbs .phi..omicron..beta..epsilon..iota..nu. and .phi..omicron..beta..epsilon..iota..sigma..theta..alpha.i which are meant here in the sense of to be afraid of or to avoid) proteins represent more than 20% of the total soluble cytoplasmic proteins and include some of the most abundant soluble cellular components. They may be recovered from high-speed supernatant (S-100) fractions and, following sucrose gradient centrifugation, most of them appear in the form of complexes smaller than 8.5S. On denaturation in urea and 2-dimensional gel electrophoresis these proteins appear to be comprised of polypeptides of widely different sizes (ca [circa] MW 15,000-230,000) and isoelectric points covering a broad range of pH values (4.2-8.0). Gel filtration and isoelectric focusing of native karyophobic proteins show that the majority occur in acidic complexes smaller than MW 150,000, including one case of a small karyophobic protein (C9; MW 30,000). In contrast to karyophilic proteins and proteins equilibrating between nucleus and cytoplasm karyophobic soluble proteins from [35S]Met-labeled ooplasms, when injected into unlabeled oocytes, remain in the cytoplasm. Human proteins with a similar karyophobic behavior were identified in fractions of soluble proteins from HeLa cells; there, the major karyophobic protein (HCa, MW 36,000) is also one of the most abundant soluble proteins. The specific nucleocytoplasmic comparmentalization of soluble proteins is governed not only by the principles of exclusion of large molecules from nuclear uptake and the existence of karyophilic signals in certain proteins but that a series of soluble, globular proteins exist in the cytoplasm, which have other molecular features which selectively exclude them from distribution over the nucleus. The possible functional role of the selective enrichment of these abundant proteins, which so far have escaped attention, in establishing a cytoplasmic milieu is discussed.