ULK1

ULK1 is an enzyme that in humans is encoded by the ULK1 gene.^[5][6]

Unc-51-like autophagy-activating kinases 1 and 2 (ULK1/2) are two similar isoforms of an enzyme that in humans is encoded by the ULK1/2 genes.^[7][8] The enzyme is specifically a kinase that is involved with autophagy, particularly in response to amino acid withdrawal. Not many studies have been done comparing the two isoforms, but some differences have been recorded.^[9]

Function

Ulk1/2 is an important protein in autophagy for mammalian cells, and is homologous to ATG1 in yeast. It is part of the ULK1-complex, which is needed in early steps of autophagosome biogenesis. The ULK1 complex also consists of the FAK family kinase interacting protein of 200 kDa (FIP200 or RB1CC1) and the HORMA (Hop/Rev7/Mad2) domain-containing proteins ATG13 and ATG101.^[10] ULK1, specifically, appears to be the most essential for autophagy and is activated under conditions of nutrient deprivation by several upstream signals which is followed by the initiation of autophagy.^[11] However, ULK1 and ULK2 show high functional redundancy; studies have shown that ULK2 can compensate for the loss of ULK1. Nutrient dependent autophagy is only fully inhibited if both ULK1 and ULK2 are knocked out.

ULK1 has many downstream phosphorylation targets to aid in this induction of the isolation membrane/ autophagosome. Recently, a mechanism for autophagy has been elucidated. Models have proposed that the active ULK1 directly phosphorylates

Ulk1/2 is negatively regulated by mTORC1 activity, which is active during anabolic-type environmental cues. In contrast, Ulk1/2 is activated by AMPK activity from starvation signals.^[13]

Ulk1/2 may have critical roles beyond what ATG1 performs in yeast, including neural growth and development.

Interactions

When active, mTORC1 inhibits autophagy by phosphorylating both ULK1 and ATG13, which reduces the kinase activity of ULK1. Under starvation conditions, mTORC1 is inhibited and dissociates from ULK1 allowing it to become active. AMPK is activated when intracellular AMP increases under starvation conditions, which inactivate mTORC1, and thus indirectly activate ULK1. AMPK also directly phosphorylates ULK1 at multiple sites in the linker region between the kinase and C-terminal domains.^[10]

ULK1 can phosphorylate itself as well as ATG13 and RB1CC1, which are regulatory proteins; however, the direct substrate of ULK1 has not been identified although recent studies suggest it phosphorylates Beclin-1.^{[citation needed]}

Upon proteotoxic stresses, ULK1 has been found to phosphorylate the adaptor protein p62/SQSTM1, which increases the binding affinity of p62/SQSTM1 for ubiquitin.^[10][14]

ULK1 has been shown to

ULK1 is phosphorylated by AMPK on Ser317 and Ser777 to activate autophagy; mTOR participates in inhibitory phosphorylation of ULK1 on Ser757.[18] Additionally, ULK1 can auto-phosphorylate itself at Thr180 to facilitate self activation.^[19]

Viral targeting of ULK1 appears to disrupt host autophagy. Coxsackievirus B3 viral proteinase 3C can proteolytically process ULK1 by cleaving after glutamine (Q) residue 524, separating the N-terminal kinase domain from C-terminal early autophagy targeting/tethering (EAT) domain.^[20]

Related diseases

Given ULK1's role in autophagy, many diseases such as cancer,^[21] neurodegenerative disorders, neurodevelopment disorders,^[22] and Crohn's disease^[23] could be attributed to any impairments in autophagy regulation.

In cancer specifically, ULK1 has become an attractive therapeutic target.^{[citation needed]} Since autophagy acts as a cell survival trait for cells, it enables tumors (once they are already formed) to survive energy deprivation and other stresses such as chemotherapeutics. For that reason, inhibiting autophagy may prove to be beneficial. Thus, inhibitors have been targeted towards ULK1.^[24]

References