Poster: Protein targeting & vesicular trafficking

Abs # 665: A maize ribosome-associated membrane protein (RAMP4) may link the ER stress response and protein translocation

Presenter:	Sreenivasa Rao, T.J.V. , jvrao@unity.ncsu.edu
Authors	Sreenivasa Rao, T.J.V.  (A)   Jung, Rudolf  (B)   Boston, Rebecca S (A)
Affiliations:	(A): Dept. of Botany, North Carolina State University, Raleigh, NC (B): Pioneer Hi-Bred International, Inc. Johnston, IA

Synthesis of functional secretory proteins requires the coordinated interaction of several multi-subunit protein complexes: ribosomes that associate with ER, the translocation apparatus, and molecular chaperones for protein folding and assembly. Perturbation of the secretory pathway by accumulation of mutant or foreign proteins in the ER induces a cellular response called the ER stress response. This response is characterized in part by induction of molecular chaperones and reversal of protein translocation for ER associated degradation. The coordination of these cellular processes in response to ER stress must require intracellular communication but the proteins responsible for such crosstalk are not known. We have identified a maize homolog of RAMP4, a 69 amino acid protein that appears to have roles in both protein translocation and ER stress. RAMP4 is associated with RER and is overproduced during pharmacological induction of ER stress and by accumulation of misfolded mutant proteins. RNA profiling experiments indicated that RAMP4 is encoded by a small gene family expressed differentially in a variety of tissues. We have characterized RAMP4 and its association with RER during normal endosperm development and in mutants exhibiting an endosperm-specific ER stress response. RAMP4 is tightly associated with both cisternal and protein body RER but preferentially associates with those subdomains undergoing ER stress. Investigation of RAMP4 interactions with RER showed dissociation by puromycin or GTP. GTP hydrolysis, however, was not required for RAMP4 dissociation. The higher levels of RAMP4 during ER stress and its tight association with ribosomes are suggestive of crosstalk between the ER stress response and the translocation machinery.