LUBAC

Linear ubiquitin chain assembly complex (LUBAC) is a multi-protein complex and the only known E3 ubiquitin ligase able to conjugate ubiquitin in a head-to-tail manner to generate linear (M1-linked) polyubiquitin chains. The complex is currently known to be composed of three proteins: heme-oxidized IRP2 ubiquitin ligase 1 (HOIL-1), HOIL-1-interacting protein (HOIP), and Shank-associated RH domain-interacting protein (SHARPIN)^{[1], [2],}.^[3] HOIL-1 and HOIP are both E3 ubiquitin ligases, however, the specific linear ubiquitin-ligating activity is enacted by HOIP.^[4] Mice deficient in HOIP are embryonically lethal.^[5] Two cases of mutated HOIP have been detected in humans. These patients presented with autoinflammation and immunodeficiency^[6],.^[7] HOIL-1 is required for LUBAC assembly and stability as demonstrated by embryonic lethality in HOIL-1 deficient mice.^[8] Recently, it has been noted, that HOIL-1 is also able to catalyze formation of oxyester bonds between the C-terminus of ubiquitin and serine/threonine of substrate protein in TLR signaling.^[9] SHARPIN exhibits a significant sequence similarity to HOIL-1 and is important for LUBAC stability. Spontaneous point mutation in the Sharpin gene in mice leads to development of chronic proliferative dermatitis (cpdm)^[10],.^[11] Both HOIL-1 and SHARPIN bind to HOIP through their ubiquitin-like (UBL) domain^[1],.^[2] LUBAC consisting of either HOIP-HOIL-1 or HOIP-SHARPIN is functional in vitro, however the greatest activity of the complex has been observed in the presence of all three components.^[2]

LUBAC modulates signaling complexes activating the canonical NF-kB pathway in response to various stimuli (e.g., TNF, IL-1, CD40L) by adding M1-linked polyubiquitin chains to signaling proteins^[2],.^[12] Additionally, LUBAC has been shown to interact with PKC and NLRP3/ASC inflammasome^[13],.^[14]

Antagonistic to LUBAC are deubiquitinases such as OTULIN or CYLD, of which OTULIN is the only deubiquitinase that removes M1-linked ubiquitin linkages exclusively.^[15]

LUBAC components have been most widely studied in the context of TNF signaling.^{[citation needed]}