Intracellular Transport of a Karyophilic Protein

Abstract

The term ‘karyophilic’ was suggested (Dabauvalle and Franke, 1982) for cellular proteins which accumulate in the nucleus without binding to chromatin or other nuclear structures. Absence of binding means that these proteins are recovered from the supernatant of centrifuged cell homogenates. The classification originated from studies of Xenopus oocytes (Bonner, 1975;; Merriam and Hill, 1976; De Robertis et al., 1978). The exceptionally large size of these cells permits manipulations not easily applied to average-sized somatic cells. For instance, Xenopus oocytes can be easily loaded with macromolecules by microinjection, they can be fractioned by particularly fast and mild manual procedures, frequently the fractions of even single cells are large enough to be analyzed by centrifugation and gel electrophoresis. With these methods several karyophilic proteins of the Xenopus oocyte were characterized and designated as N1-N4 and nucleoplasmin. Karyophilic proteins are also present in oocytes from amphibians other than Xenopus (Dabauvalle and Franke, 1982). However, their occurance in somatic cells is largely unresolved.