Evidence for export of a muscle lectin from cytosol to extracellular matrix and for a novel secretory mechanism.

TLDR

A soluble lactose‑binding lectin of 14,500 Da is detected extracellularly but lacks a secretion signal, raising questions about its export mechanism. The study aims to elucidate how this cytosolic lectin is exported to the extracellular matrix. The authors propose that export occurs via plasma‑membrane evaginations that pinch off as lectin‑rich extracellular vesicles. They demonstrated that the lectin is externally secreted from C2 mouse muscle cells in a developmentally regulated manner, co‑localizing with laminin and concentrating in membrane evaginations that form labile extracellular vesicles.

Abstract

A soluble lactose-binding lectin with subunit Mr of 14,500 is believed to function by interacting with extracellular glycoconjugates, because it has been detected extracellularly by immunohistochemistry. This localization has been questioned, however, since the lectin lacks a secretion signal sequence, which challenges the contention that it is secreted. We have demonstrated externalization of this lectin from C2 mouse muscle cells by both immunoprecipitation of metabolically labeled protein and immunohistochemical localization. We further show that externalization of the lectin is a developmentally regulated process that accompanies myoblast differentiation and that the lectin codistributes with laminin in myotube extracellular matrix. Immunohistochemical localization during intermediate stages of externalization suggests that the lectin becomes concentrated in evaginations of plasma membrane, which pinch off to form labile lectin-rich extracellular vesicles. This suggests a possible mechanism for lectin export from the cytosol to the extracellular matrix.