Conserved Pro-Glu (PE) and Pro-Pro-Glu (PPE) Protein Domains Target LipY Lipases of Pathogenic Mycobacteria to the Cell Surface via the ESX-5 Pathway

Maria H Daleke,Alessandro Cascioferro,Karin De Punder,Roy Ummels,Abdallah M Abdallah,Nicole Van Der Wel,Peter J Peters,Joen Luirink,Riccardo Manganelli,Wilbert Bitter

doi:10.1074/jbc.m110.204966

Abstract

The type VII secretion system ESX-5 is a major pathway for export of PE and PPE proteins in pathogenic mycobacteria. These mycobacteria-specific protein families are characterized by conserved N-terminal domains of 100 and 180 amino acids, which contain the proline-glutamic acid (PE) and proline-proline-glutamic acid (PPE) motifs after which they are named. Here we investigated secretion of the triacylglycerol lipase LipY, which in fast-growing mycobacteria contains a signal sequence, but in slow-growing species appears to have replaced the signal peptide with a PE or PPE domain. Selected LipY homologues were expressed in wild-type Mycobacterium marinum and its corresponding ESX-5 mutant, and localization of the proteins was investigated by immunoblotting and electron microscopy. Our study shows that Mycobacterium tuberculosis PE-LipY (LipY(tub)) and M. marinum PPE-LipY (LipY(mar)) are both secreted to the bacterial surface in an ESX-5-dependent fashion. After transport, the PE/PPE domains are removed by proteolytic cleavage. In contrast, Mycobacterium gilvum LipY, which has a signal sequence, is not transported to the cell surface. Furthermore, we show that LipY(tub) and LipY(mar) require their respective PE and PPE domains for ESX-5-dependent secretion. The role of the PE domain in ESX-5 secretion was confirmed in a whole cell lipase assay, in which wild-type bacteria expressing full-length LipY(tub), but not LipY(tub) lacking its PE domain, were shown to hydrolyze extracellular lipids. In conclusion, both PE and PPE domains contain a signal required for secretion of LipY by the ESX-5 system, and these domains are proteolytically removed upon translocation.

Highlights

Mycobacteria such as Mycobacterium tuberculosis, the etiological agent of tuberculosis, have a highly unusual and complex cell envelope [1]
Homologues of LipY Have Evolved Different N-terminal Domains—M. tuberculosis LipY consists of three domains: a typical proline-glutamic acid (PE) domain at the N terminus followed by a linker domain of unknown function and a C-terminal domain containing the triacylglycerol lipase motif
Comparative genomics and phylogenetic analyses indicated that the evolution of the type VII secretion system ESX-5 predated the expansion of the PE and proline-proline-glutamic acid (PPE) gene families in slowgrowing mycobacteria [18]

Summary

Introduction

Mycobacteria such as Mycobacterium tuberculosis, the etiological agent of tuberculosis, have a highly unusual and complex cell envelope [1]. Because LipYtub and LipYmar lacking their N-terminal PE or PPE domains have been shown to have higher lipase activity than the corresponding full-length proteins [33], these results confirm that our N-terminally truncated LipY homologues are not secreted.

Results

Conclusion