Reconstituting the Mammalian Apoptotic Switch in Yeast

Abstract

Proteins of the Bcl-2 family regulate the permeabilization of the mitochondrial outer membrane that represents a crucial irreversible step in the process of induction of apoptosis in mammalian cells. The family consists of both proapoptotic proteins that facilitate the membrane permeabilization and antiapoptotic proteins that prevent it in the absence of an apoptotic signal. The molecular mechanisms, by which these proteins interact with each other and with the mitochondrial membranes, however, remain under dispute. Although yeast do not have apparent homologues of these apoptotic regulators, yeast cells expressing mammalian members of the Bcl-2 family have proved to be a valuable model system, in which action of these proteins can be effectively studied. This review focuses on modeling the activity of proapoptotic as well as antiapoptotic proteins of the Bcl-2 family in yeast.

Keywords: BH3-only proteins; Bax; Bcl-2 protein family; Bcl-XL; Saccharomyces cerevisiae; apoptosis; mitochondria.

Figure 1

Mitochondrial (intrinsic) apoptotic pathway. Cell death-inducing signals are processed by proteins of the Bcl-2 family. BH3-only proteins in reaction to these signals activate Bax and Bak to permeabilize the outer mitochondrial membrane by either direct interaction or by inhibition of antiapoptotic proteins (e.g., Bcl-XL, Bcl-2). Cytochrome c escapes the permeabilized mitochondria to induce the assembly of apoptosome and activation of caspases, which ultimately results in cell death by apoptosis. The part of a pathway, reconstitution of which in yeast is a subject of this review, is outlined by light blue silhouette of budding yeast cell. Green silhouette of yeast cell outlines a part of the pathway that has been separately reconstituted in yeast [5] but is outside the scope of this review.

Figure 2

The Bcl-2 family of proteins. Domain structure of Bcl-2 family proteins is presented schematically. Sizes of proteins and domains are roughly in scale and position of BH domains is indicated. In multidomain proteins, positions of α-helices are indicated with numbered black bars.

Figure 3

Two modes of Bcl-2 family proteins action observed in yeast model systems. (a) Bax or Bak are expressed as constitutively active cells killing proteins. Their activity is inhibited by coexpression of antiapoptotic protein (Bcl-XL, Bcl-2, Bcl-w). Coexpressed BH3-only proteins inhibit Bax-inhibiting activity of antiapoptotic proteins, resulting in active Bax or Bak. (e.g., [65]) (b) Bax requires activation by BH3-only proteins to become an active cells killing protein. (Such activation can also be achieved by introducing a point mutation into Bax). Bcl-XL appears to both promote mitochondrial targeting of Bax and inhibition of pore formation [63].