The assembly of β‐barrel membrane proteins by BAM and SAM. Issue 3 (23rd December 2020)

• Record Type: Journal Article
• Authors: Lundquist, Karl; Billings, Evan; Bi, Maxine; Wellnitz, James; Noinaj, Nicholas
• Other Names: Blokesch Melanie guestEditor.; Palmer Tracy guestEditor.
• Is Part Of: Molecular microbiology. Volume 115:Issue 3(2021)
• Page Start: 425
• Page End: 435
• Publication Date: 2020-12-23
• Subjects: envelope biogenesis -- Gram‐negative bacteria -- lateral gate -- outer membrane -- outer membrane protein -- protein folding -- β‐barrel; Molecular microbiology -- Periodicals; 572.829
• DOI: 10.1111/mmi.14666
• Languages: English
• ISSNs: 0950-382X

Abstract

Abstract: Gram‐negative bacteria, mitochondria, and chloroplasts all possess an outer membrane populated with a host of β‐barrel outer‐membrane proteins (βOMPs). These βOMPs play crucial roles in maintaining viability of their hosts, and therefore, it is essential to understand the biogenesis of this class of membrane proteins. In recent years, significant structural and functional advancements have been made toward elucidating this process, which is mediated by the β‐barrel assembly machinery (BAM) in Gram‐negative bacteria, and by the sorting and assembly machinery (SAM) in mitochondria. Structures of both BAM and SAM have now been reported, allowing a comparison and dissection of the two machineries, with other studies reporting on functional aspects of each. Together, these new insights provide compelling support for the proposed budding mechanism, where each nascent βOMP forms a hybrid‐barrel intermediate with BAM/SAM in route to its biogenesis into the membrane. Here, we will review these recent studies and highlight their contributions toward understanding βOMP biogenesis in Gram‐negative bacteria and in mitochondria. We will also weigh the evidence supporting each of the two leading mechanistic models for how BAM/SAM function, and offer an outlook on future studies within the field. Abstract : The biogenesis of b‐barrel outer membrane proteins (bOMPs) is mediated by the BAM and SAM complexes in Gram‐negative bacteria and mitochondria, respectively. Studies have reported the structures of both of these complexes and functional characterization has demonstrated both BAM and SAM open laterally within the membrane. Recent evidence favors a mechanistic model where new bOMPs nucleate and bud away from the central b‐domain of BAM and SAM.