investigation on the function of Sec71 protein in Sec62/63 complex

Abstract

Membrane and secretory proteins are synthesized by ribosomes in cytoplasm and targeted to the endoplasmic reticulum (ER) membrane via two pathways: co translational translocation pathway and post translational translocation pathway. In case of post translational translocation pathway it has been reported that Sec61 works with distinct hetero-tetrameric protein complex called Sec62/Sec63 complex. Sec63 complex consists of Sec62, Sec63 essential proteins and Sec71, Sec72 which are dispensable. Sec62p was previously shown to play a role on the translocation of moderately hydrophobic single and multi spanning membrane proteins. Since Sec71p and Sec62p belong to the same Sec62/Sec63 complex, we were curious about the role of Sec71 on the translocation of membrane proteins. To investigate the function of Sec71 in translocation and or insertion of secretory and membrane proteins, a deletion strain lacking SEC71 was constructed. When a set of model proteins was expressed in sec71::HIS3 (sec71 deletion strain), the translocation of some presecretory proteins was impaired. In case of membrane proteins, the effect of SEC71 deletion was more intense in case of single spanning (Lep-H1 model protein) where sec71 deletion abolished the targeting of moderately hydrophobic proteins and still attenuated the targeting even for the ones having a higher hydrophobicity. The multi spanning membrane proteins (H2 and H3) were affected only with the moderately hydrophobic TMD which insertion was decreased in absence of Sec71p. Our results on translocation and membrane insertion profiles of single spanning (H1) and multispanning membrane proteins (H2 and H3) in sec71 deletion cell suggest two possibilities: the Sec71 may be responsible for the stability of the Sec62/Sec63 complex or Sec71p may stabilize directly the translocating proteins which are less hydrophobic or large which may require more energy to be translocated across the ER. Sec71p may with some interactions with other translocation components help these proteins to position better for optimal translocation and membrane insertion.