Mycobacterial methionine aminopeptidase type 1c moonlights as an anti-association factor on the 30S ribosomal subunit

Methionine aminopeptidase (MetAP), an essential metalloprotease, binds the 70S ribosome near the exit site of peptide tunnel and cleaves the N-terminal methionine from the nascent polypeptide chains. While E. coli expresses a single subclass of type1 MetAP, mycobacteria possesses two subclasses, namely, MetAP1a and MetAP1c, differentiated by the presence of a 40 amino acid long N-terminal tether in the later. Here we reveal a yet unknown moonlighting role of MetAP1c in mycobacteria. We observe stationary phase-specific enhancement of expression and a unique 30S subunit affinity of MetAP1c. A 5[A] cryo-EM map of Mycobacterium smegmatis (Msm) MetAP1c-30S subunit complex manifests for the first time the binding of MetAP1c at the inter-subunit side in the vicinity of proteins S13 and S19. MetAP1c binding and resulting conformational changes make the 30S subunit unfit for association with the 50S subunit, thereby conferring an anti-association property on MetAP1c. We further constructed two N-terminal mutants to establish the pivotal role of the N-terminal extension of MetAP1c in its moonlighting function.