The motors of protein import into chloroplasts

Abstract

Chloroplast function is largely dependent on its resident proteins, most of which are encoded by the nuclear genome and are synthesized in cytosol. Almost all of these are imported through the translocons located in the outer and inner chloroplast envelope membranes. The motor protein that provides the driving force for protein import has been proposed to be Hsp93, a member of the Hsp100 family of chaperones residing in the stroma. Combining in vivo and in vitro approaches, recent publications have provided multiple lines of evidence demonstrating that a stromal Hsp70 system is also involved in protein import into this organelle. Thus it appears that protein import into chloroplasts is driven by two motor proteins, Hsp93 and Hsp70. A perspective on collaboration between these two chaperones is discussed.

Figure 1

Concurrent with or after translocating across outer envelope membrane, the precursor protein is engaged in the Tic transport channel. While the precursor is in the channel, it interacts with a stromal motor complex that is minimally composed of Tic110, Tic40, Hsp93 and Hsp70. The motor pulls the precursor into stromal at expense of AT P hydrolysis. The two AT Pases, Hsp93 and Hsp70, may cooperate in two different modes: (A) sequentially and independently or (B) simultaneously and synergistically (refer to text for details). Hsp70 associates with and dissociates from the motor complex under regulation of its two co-chaperones, CGE and JDP, to complete an AT P hydrolysis cycle. The transit peptide of the importing precursor is removed by the stromal processing peptidase (SPP).