Autophagy-independent function of MAP-LC3 during intracellular propagation of Chlamydia trachomatis

Microtubule-associated protein 1 (MAP1) light chain 3 (LC3) has proven useful as autophagosomal marker in studies on the interaction between pathogens and the host autophagic machinery. However, the function of LC3 is known to extend above and beyond its role in autophagosome formation. We previously reported that intrinsic LC3 is associated with the intracellular Chlamydia trachomatis inclusion in human epithelial cells. Here we show that LC3, most likely the cytoplasmic nonlipidated form, interacts with the C. trachomatis inclusion as a microtubule-associated protein rather than an autophagosome-associated component. In contrast, N-terminally GFP-tagged LC3 exclusively targets autophagosomes rather than chlamydial inclusions. Immunofluorescence analysis revealed an association of LC3 and MAP1 subunits A and B with the inclusion as early as 18 h post infection. Inclusion-bound LC3 was connected with the microtubular network. Depolymerization of the microtubular architecture disrupted the association of LC3/MAP1s with the inclusion. Furthermore, siRNA-mediated silencing of the MAP1 and LC3 proteins revealed their essential function in the intracellular growth of C. trachomatis. Interestingly, defective autophagy remarkably enhanced chlamydial growth, suggesting a suppressive effect of the autophagic machinery on bacterial development. However, depletion of LC3 in autophagy-deficient cells noticeably reduced chlamydial propagation. Thus, our findings demonstrate a new function for LC3, distinct from autophagy, in intracellular bacterial pathogenesis.