Identification of the versatile scaffold protein RACK1 on the eukaryotic ribosome by cryo-EM

Abstract

RACK1 serves as a scaffold protein for a wide range of kinases and membrane-bound receptors. It is a WD-repeat family protein and is predicted to have a beta-propeller architecture with seven blades like a Gbeta protein. Mass spectrometry studies have identified its association with the small subunit of eukaryotic ribosomes and, most recently, it has been shown to regulate initiation by recruiting protein kinase C to the 40S subunit. Here we present the results of a cryo-EM study of the 80S ribosome that positively locate RACK1 on the head region of the 40S subunit, in the immediate vicinity of the mRNA exit channel. One face of RACK1 exposes the WD-repeats as a platform for interactions with kinases and receptors. Using this platform, RACK1 can recruit other proteins to the ribosome.

Translational control by 80S formation and 60S availability: The central role of eIF6, a rate limiting factor in cell cycle progression and tumorigenesis

“…24 This model places eIF6 activity downstream of the growth factor/PKC signaling cascade and explains observations indicating that eIF6 can repress translation in vitro but not in vivo when ras/PKC stimulation is performed (see below). The model fits data showing that PKC binds RACK1, [25][26][27] and its stimulation regulates translation in many conditions.…”

“…The model fits data showing that PKC binds RACK1, [25][26][27] and its stimulation regulates translation in many conditions. 25,28,29 However, although RACK1 binds cytoplasmic 40S subunits, its location at the mRNA exit channel of 40S places it far from the intersubunit bridges, 24 and PKC signaling is not evolutionarily conserved in yeasts.…”

Rack1 protects N-terminal phosphorylated c-Jun from Fbw7-mediated degradation

“…Through its WD40 domains, Rack1 can bind one or two other molecules to form a complex that has an important role in various cellular functions (Schechtman and Mochly-Rosen, 2001). For example, Rack1 functions as an adaptor protein between activated protein kinase C and JNKs to enhance JNKs phosphorylation (Lopez-Bergami et al, 2005) and regulates translation as a component of the 80S ribosome (Sengupta et al, 2004). Rack1 also promotes degradation of HIF1a (Liu et al, 2007) and BimEL (Zhang et al, 2008) by mediating the interaction between each of these proteins and the ubiquitin complex.…”

Eukaryote-specific extensions in ribosomal proteins of the small subunit: Structure and function

“…Asc1 is an ortholog of mammalian RACK1 which belongs to the WD-repeat protein family exhibiting a 7 bladed b-propeller structure. 6,73 RACK1 is known to play an important role in numerous biological responses/signaling events, in turn involving its interaction with kinases and receptors in a regulated manner. 74 Therefore, interaction between uS3 and Asc1 might act as an important link between translational control of gene expression and cellular signaling.…”