Kinase Family BLVRA
Kinase Classification: Group SAPPK: Family BLVRA
Best known as the enzyme that reduces biliverdin to bilrubin, as part of bile acid production and a possible celular redox cycle. BLVR (BVR; billiverdin reductase) has also been implicated in Insulin Receptor signaling, MAPK activity, PKC activation and even as a transcription factor, though some kinase activities may be due to its binding to kinases such as FAK [1].
Reported Kinase Activity
Autophosphorylation activity was demonstrated by Salim et al [2], using BLVRA purified by GST affinity chromatography from E. coli. An in-gel assay of a renatured prep (not clear whether purified from E. coli or human source) also showed autophosphorylation activity. The pH optimum of 8.7 was unusually high. BLVRA bound the fluorescent ATP analog TNP-ATP, at a similar pH optimum, and contains a nucleotide-binding Rossman fold (known to beind NADPH during the reductase activity [3]. A low level of transphosphorylation of casein was measured at 2.7 pmol/mg/min, and some phosphorylation of MBP was also seen. The protein is autophosphorylated on both Ser and Tyr. Some Ser phosphorylation is required for full reductase activity, and protein phosphorylatin was induced by hydrogen peroxide, a putative activator of the reductase function. Mutation of Y72 or Y83 reduced the observed tyrosine autophosphorylation, suggesting that these are autophosphorylation sites. These tyrosines are conserved only in the mammalian homologs of BLVRA.
Additional papers from the same group show that three tyrosines on BLVRA (Y198, Y228, Y291) are phosphorylated in vitro by activated insulin receptor. The sequence surrounding Y198 is an optimal substrate motif for insulin receptor kinase activity and SH2 binding. This phosphorylation site was confirmed by mass spec analysis. The other two tyrosines are not well conserved even within mammals, and were identified only by reduced phosphorylation in point mutants.
Phosphoproteomic studies (Phosphosite) have found only S320 (partially conserved in mammals) as a phosphosite, casting some doubt on the in vivo significance of the tyrosine phosphorylation of BLVRA.
Other Signaling Activities
BLVRA was shown to be important for neuronal plasticity and synapse function in mice, and this function was linked to a role for BLVRA in allowing FAK/Pyk2 to bind and activate the SRC kinase [4]
Evolution
BLVRA is a single-copy gene in all vertebrates, with a well conserved sequence (55% identity between Danio and human). It is lost from most metazoan lineages, including invertebrate chordates, arthropods and nematodes, but present in several cnidarians and lophotrochozoa. Homologs are also widespread in bacteria (at ~25% identity). An unrelated but equivalent enzyme, biliverdin reductase B (BLVRB) is more widespread in animals.
Domain Structure
BLVRA has a billiverdin reductase catalytic domain followed by an oxidoreductase domain. It has no known kinase domain.
Functions
The primary function of BLVRA is to reduce biliverdin to bilirubin, a byproduct of heme catabolism, and redox agent.
References
- Vasavda C, Semenza ER, Liew J, Kothari R, Dhindsa RS, Shanmukha S, Lin A, Tokhunts R, Ricco C, Snowman AM, Albacarys L, Pastore F, Ripoli C, Grassi C, Barone E, Kornberg MD, Dong X, Paul BD, and Snyder SH. Biliverdin reductase bridges focal adhesion kinase to Src to modulate synaptic signaling. Sci Signal. 2022 May 10;15(733):eabh3066. DOI:10.1126/scisignal.abh3066 |
- Salim M, Brown-Kipphut BA, and Maines MD. Human biliverdin reductase is autophosphorylated, and phosphorylation is required for bilirubin formation. J Biol Chem. 2001 Apr 6;276(14):10929-34. DOI:10.1074/jbc.M010753200 |
- Whitby FG, Phillips JD, Hill CP, McCoubrey W, and Maines MD. Crystal structure of a biliverdin IXalpha reductase enzyme-cofactor complex. J Mol Biol. 2002 Jun 21;319(5):1199-210. DOI:10.1016/S0022-2836(02)00383-2 |
