LC3-dependent intracellular membrane tubules induced by γ-protocadherins A3 and B2: A role for intraluminal interactions

Clustered protocadherins (Pcdhs) are a family of cadherin-like molecules arranged in gene clusters (α, β, and γ). γ-Protocadherins (Pcdh-γs) are involved in cell-cell interactions, but their prominent intracellular distribution in vivo and different knock-out phenotypes suggest that these molecules participate in still unidentified processes. We found using correlative light and electron microscopy that Pcdh-γA3 and -γB2, but not -γC4, -α1, or N-cadherin, generate intracellular juxtanuclear membrane tubules when expressed in cells. These tubules recruit the autophagy marker MAP1A/1B LC3 (LC3) but are not associated with autophagic vesicles. Lipidation of LC3 is required for its coclustering with Pcdh-γtubules, suggesting the involvement of an autophagic-like molecular cascade. Expression of wild-type LC3 with Pcdh-γA3 increased tubule length whereas expression of lipidation-defective LC3 decreased tubule length relative to Pcdh-γA3 expressed alone. The tubules were found to emanate from lysosomes. Deletion of the luminal/extracellular domain of Pcdh-γA3 preserved lysosomal targeting but eliminated tubule formation whereas cytoplasmic deletion eliminated both lysosomal targeting and tubule formation. Deletion of the membrane-proximal three cadherin repeats resulted in tubes that were narrower than those produced by full-length molecules. These results suggest that Pcdh-γA and -γB families can influence the shape of intracellular membranes by mediating intraluminal interactions within organelles.