Locations of membrane protein production in a cyanobacterium

Abstract

Cyanobacteria show an unusually complex prokaryotic cell structure including a distinct intracytoplasmic membrane system, the thylakoid membranes that are the site of the photosynthetic light reactions. The thylakoid and plasma membranes have sharply distinct proteomes, but the mechanisms that target proteins to a specificmembrane remain poorly understood. Here, we investigate the locations of translation of thylakoid and plasma membrane proteins in the model unicellular cyanobacterium Synechococcus elongatus PCC 7942. We use fluorescentin situ hybridization to probe the locations of mRNAs encoding membrane-integral proteins, plus Green Fluorescent Protein tagging of the RplL subunit to reveal the location of ribosomes under differentconditions. We show that membrane-integral thylakoid and plasma membrane proteins are translated in differentlocations. Thylakoid membrane proteins are translated in patches at the innermost thylakoid membrane surface facing the nucleoid. However, differentproteins are translated in differentpatches, even when they are subunits of the same multiprotein complex. This implies that translation is distributed over the proximal thylakoid surface, with newly inserted proteins migrating within the membrane prior to incorporation into complexes. mRNAs encoding plasma membrane proteins form patches at the plasma membrane. Ribosomes can be observed at similar locations near the thylakoid and plasma membranes, with more ribosomes near the plasma membrane when conditions force rapid production of plasma membrane proteins. There must be routes for ribosomes and mRNAs past the thylakoids to the plasma membrane. We infer a system to chaperone plasma membrane mRNAs to prevent their translation prior to arrival at the correct membrane.