Unconventional protein secretion: an evolving mechanism

The process by which proteins are secreted without entering the classical endoplasmic reticulum (ER)-Golgi complex pathway, in eukaryotic cells, is conveniently called unconventional protein secretion. Recent studies on one such protein called Acb1 have revealed a number of components involved in its secretion. Interestingly, conditions that promote the secretion of Acb1 trigger the biogenesis of a new compartment called CUPS (Compartment for Unconventional Protein Secretion). CUPS form near the ER exit site but lack ER-specific proteins. Other proteins that share some of the features common with the secretion of Acb1 are interleukin-1 beta and tissue transglutaminase. Here I will review recent advances made in the field and propose a new model for unconventional protein secretion.