Killing of Microbes and Cancer by the Immune System with Three Mammalian Pore-Forming Killer Proteins

Abstract

Immunology is the science of biological warfare between the defenses of our immune systems and offensive pathogenic microbes and cancers. Over the course of his scientific career, Eckhard R. Podack made several seminal discoveries that elucidated key aspects of this warfare at a molecular level. When Eckhard joined the complement laboratory of Müller-Eberhard in 1974, he was fascinated by two questions: (1) what is the molecular mechanism by which complement kills invasive bacteria? and (2) which one of the complement components is the killer molecule? Eckhard's quest to answer these questions would lead to the discovery C9 and later, two additional pore-forming killer molecules of the immune system. Here is a brief account of how he discovered poly-C9, the pore-forming protein of complement in blood and interstitial fluids: Perforin-1, expressed by natural killer cells and cytotoxic T lymphocytes; and Perforin-2 (MPEG1), expressed by all cell types examined to date. All the three killing systems are crucial for our survival and health.

Keywords: C9; MPEG1; Perforin-1; Perforin-2; complement; cytotoxic T cells; natural killer cells; pore-forming protein.

Figure 1

Domain organization of C9, Perforin-1, and Perforin-2. All the three pore-forming proteins contain amino-terminal signal peptides (gray cylinders) and membrane attack complex perforin (MACPF) domains. Unlike the soluble proteins C9 and Perforin-1, Perforin-2 is a type I membrane protein with a membrane spanning alpha helix (black cylinder) near its carboxy terminus. The short cytosolic tail (wavy line) of Perforin-2 is ubiquitylated by a CRL in response to PAMPs such as LPS. Perforin-2 also contains a domain of unknown function (DUF) that is conserved among Perforin-2 orthologs. Domain architecture was retrieved from UniProt entries P02748, P14222, and Q2M385. Abbreviations: TSP1, thrombospondin type-1 repeat; LDL, low-density lipoprotein receptor class A repeat; EGF, epidermal growth factor-like domain; C2, calcium-dependent phospholipid-binding domain. This figure was adapted from McCormack et al. (1).

Figure 2

Structure of poly-C9 at 8 Å resolution. Side, tilted, and top views of soluble poly-C9 obtained by cryo-EM. In this reconstruction, 22 C9 molecules polymerize to form a symmetrical structure with globular domains atop a membrane penetrating 88 stranded β-barrel with a diameter of 120 Å. Graphic representations were derived from PDB file 5FMW and rendered with NGL Viewer (27).