Degradation of proteins microinjected into cultured mammalian cells

Abstract

Iodinated proteins were degraded after injection into HeLa cells at first-order rates with half-lives varying from three hours for the trout nonhistone chromosomal protein, HMG-T, to 60 hours for whale myoglobin. Fluoresceinated-bovine serum albumin (fl-BSA) was degraded almost twice as fast as unmodified BSA. The rate of degradation of ¹²⁵I-BSA was very similar in eight cell lines of mouse, human, monkey and rat origin. Microinjected proteins were analyzed on SDS-acrylamide gels after injection, and for BSA and immunoglobin G, all remaining intracellular ¹²⁵I migrated at the molecular weight of the injected proteins. By contrast, more than 80% of the extracellular ¹²⁵I chromatographed as iodotyrosine. With the exception of fl-BSA, which exhibited perinuclear accumulation in approximately one-half of the injected cells, autoradiography showed that throughout the period of study the injected proteins remained dispersed in the cytoplasm.