Epithelial protein lost in neoplasm (EPLIN), an actin-binding protein, has been described as both a tumor promoter and tumor suppressor in different cancers. The roles of EPLIN isoforms (α/β) remain l Show more
Epithelial protein lost in neoplasm (EPLIN), an actin-binding protein, has been described as both a tumor promoter and tumor suppressor in different cancers. The roles of EPLIN isoforms (α/β) remain largely unknown and could explain these opposing views. We observed distinct EPLIN isoform localization in breast cancer cells; EPLINα is recruited to actin in plasma membrane ruffles and endosomes, while EPLINβ resides on stress fibers. EPLINα localizes to early endosomes in an actin-dependent manner, where it interacts with Rab21, an established regulator of β1-integrin endosomal trafficking. This supports β1-integrin recycling and cell migration. Using proximity biotinylation (BioID), we identified coronin 1C as an EPLIN-proximal protein, which also localizes at Rab21-containing endosomes and controls integrin recycling downstream of EPLINα. EPLINα expression was linked to increased breast cancer cell motility, and a high EPLINα-to-EPLINβ ratio correlated with a mesenchymal phenotype in patient samples. Our work identifies previously unknown EPLIN-isoform-specific functions relevant to breast cancer and beyond. Show less
Focal adhesions (FAs) connect inner workings of cell to the extracellular matrix to control cell adhesion, migration and mechanosensing. Previous studies demonstrated that FAs contain three vertical l Show more
Focal adhesions (FAs) connect inner workings of cell to the extracellular matrix to control cell adhesion, migration and mechanosensing. Previous studies demonstrated that FAs contain three vertical layers, which connect extracellular matrix to the cytoskeleton. By using super-resolution iPALM microscopy, we identify two additional nanoscale layers within FAs, specified by actin filaments bound to tropomyosin isoforms Tpm1.6 and Tpm3.2. The Tpm1.6-actin filaments, beneath the previously identified α-actinin cross-linked actin filaments, appear critical for adhesion maturation and controlled cell motility, whereas the adjacent Tpm3.2-actin filament layer beneath seems to facilitate adhesion disassembly. Mechanistically, Tpm3.2 stabilizes ACF-7/MACF1 and KANK-family proteins at adhesions, and hence targets microtubule plus-ends to FAs to catalyse their disassembly. Tpm3.2 depletion leads to disorganized microtubule network, abnormally stable FAs, and defects in tail retraction during migration. Thus, FAs are composed of distinct actin filament layers, and each may have specific roles in coupling adhesions to the cytoskeleton, or in controlling adhesion dynamics. Show less