The role of eIF4E (CDC33) in regulating early G(1) cell cycle events in Saccharomyces cerevisiae.

摘要

Translational control plays a major role in the regulation of gene expression in eukaryotic cells. New evidence has pinpointed eIF4E as a major contributor in the regulation of translational initiation events. The rate-limiting step in the translational process is at the level of initiation, where individual mRNAs are selected for protein synthesis. eIF4E binds the 7mG cap of the mRNA, and recruits other initiation factors that make up the eIF4F complex, this complex then unwinds secondary structure in the 5′ untranslated region of mRNAs in an ATP dependent reaction. This function is critical during scanning for exposing and locating the translation start site. The message is thus readied for the elongation phase of translation. Since eIF4E is known to be the least abundant of the initiation factors, this information depicts eIF4E as essential for translational initiation and the rate limiting factor in the initiation process. Many of the genes regulated at the initiation phase affect cell growth, proliferation, and differentiation. Overexpression of 4E in mammalian cells causes deregulated cell growth and malignant transformation and EIF4E is elevated in several human neoplasms. This indicates that alterations in the levels of 4E may dramatically alter the proper cell cycle progression and be an underlying cause in the tumorigenic processes. This study is aimed at understanding how increasing levels of 4E may effect cell cycle progression in the budding yeast Saccharomyces cerevisiae. I show that the overexpression of 4E (CDC33) causes an increase in the translation of the early cyclin gene CLN3. This increase in CLN3 expression affects the proper regulation of Start events in the cell by causing a premature transition from G1 to S phase of the cell cycle. This is depicted phenotypically by smaller than usual cells with increased DNA content that are slow growing. A high copy suppressor of this slow growth was the transcription factor MBP1. MBP1 is part of the transcriptional activator complex MBF. I show that overexpression of MBP1 causes an alteration in the transcriptional start site selection of CLN3, increasing the message by 45 nucleotides. This lengthened 5′ untranslated region reduces translation efficiency and downregulates CLN3 protein synthesis, thereby correcting the excess translation created by CDC33 overexpression. We therefore propose that regulation of translational initiation by eIF4E (CDC33) plays a critical role in the activation of Start and cell cycle progression in budding yeast. This information may assist in directing research designed to understand the regulatory role of 4E in the mammalian cell cycle and in the misregulation of translation in tumorigenesis.