Identification and characterization of a novel cell cycle-regulated internal ribosome entry site.

PITSLRE protein kinases are related to the large family of cyclin-dependent kinases. They have been proposed to act as tumor suppressor genes and have been shown to play a role in cell cycle progression. We report that two PITSLRE protein kinase isoforms, namely p11O(PITSLRE) and p58(PITSLRE), are translated from a single transcript by initiation at alternative in-frame AUG codons. p110(PITSLRE) is produced by classical cap-dependent translation, whereas p58(PITSLRE) results from internal initiation of translation controlled by an internal ribosome entry site (IRES) with unique properties. The IRES element is localized to the mRNA coding region, and its activity is cell cycle regulated, which permits translation of p58(PITSLRE) in G2/M.