An estimated 30% of human genes encode proteins carrying an N-terminal amino acid sequence that targets them to the ER. Most of these proteins are transported from the ER to the Golgi and are secreted from cells by an extensively studied and well-known conventional mode of protein secretion.
Rubartelli et al. (1990)
previously reported that interleukin-1β, which lacks a signal sequence for targeting to the ER, was secreted from cells. This was a surprise at the time, but since then, a large number of proteins have been found that lack a signal sequence and are secreted by eukaryotic cells. This class of proteins includes FGF2, the β-galactoside–specific lectins galectin 1 and 3, blood coagulation factor XIIIa, some members of the interleukin family, macrophage migration inhibitory factor, the engrailed homeoprotein, and the acyl coenzyme A–binding protein (ACBP), AcbA (Grundmann et al., 1988
; Lutomski et al., 1997
; Joliot et al., 1998
; Menon and Hughes, 1999
; Flieger et al., 2003
; Kinseth et al., 2007
; Nickel and Seedorf, 2008
The best-known example of an unconventionally secreted protein in the yeast Saccharomyces cerevisiae
is the mating pheromone a-factor (Kuchler et al., 1989
). After lipid modification, this protein is secreted directly across the cell surface by a dedicated ABC transporter, Ste6. However, this does not appear to be the mechanism for transport of AcbA because pharmacological inhibition of ABC transporters in Dictyostelium
does not affect secretion of this protein (Kinseth et al., 2007
). Moreover, secretion of AcbA in D.
requires the Golgi membrane–associated protein GRASP, which is not required for secretion of a-factor from yeast (Kinseth et al., 2007
). Noncanonical transport of α-integrin in Drosophila
embryos has also been shown to require dGRASP (Schotman et al., 2008
). The phosphoinositide lipid PI(4,5)P2
is necessary for the unconventional secretion mechanism of FGF2 in mammalian cells (Temmerman et al., 2008
), but little else is known about its secretory pathway.
We have analyzed the S. cerevisiae homologue of AcbA, Acb1, which also lacks a signal sequence. Surprisingly, when yeast cells are starved for nitrogen, we found that they secrete Acb1 into the medium where it is processed into signals that can stimulate rapid encapsulation of D. discoideum. Secretion of Acb1 requires autophagy genes and membrane fusion with the plasma membrane in a process that cooperates with the Golgi GRASP protein Grh1, the early endosomal compartments, and the multivesicular body (MVB) sorting pathway. The discussion of this unique secretory pathway follows.