MIC

Amino Acid Sequence

3D Structure (/structures/AF-P02769-F1-model_v6.cif)

Function

Binds water, Ca2+, Na+, K+, fatty acids, hormones, bilirubin and drugs. Its main function is the regulation of the colloidal osmotic pressure of blood. Major zinc transporter in plasma, typically binds about 80% of all plasma zinc (By similarity). Major calcium and magnesium transporter in plasma, binds approximately 45% of circulating calcium and magnesium in plasma (Probable). Potentially has more than two calcium-binding sites and might additionally bind calcium in a non-specific manner. The shared binding site between zinc and calcium at residue Asp-272 suggests a crosstalk between zinc and calcium transport in the blood (Probable). The rank order of affinity is zinc > calcium > magnesium (Probable). Binds to the bacterial siderophore enterobactin and inhibits enterobactin-mediated iron uptake of E.coli, and may thereby limit the utilization of iron and growth of enteric bacteria such as E.coli. Does not prevent iron uptake by the bacterial siderophore aerobactin.

Mechanism

The ubiquitin-proteasome pathway (UPP) constitutes the principal pathway for protein degradation in eukaryotic cells, mediating the breakdown of target proteins through ubiquitin tagging and subsequent proteasomal degradation. In an ATP-dependent manner, the ubiquitin-activating enzyme (E1) activates ubiquitin (Ub) and conjugates it to a ubiquitin-conjugating enzyme (E2). E3 ligase subsequently catalyzes the transfer of Ub to the target protein, leading to the formation of a polyubiquitin chain that completes the ubiquitination mark. The 19S regulatory particle of the 26S proteasome recognizes this ubiquitination signal and, after deubiquitination, mediates the ATP-dependent delivery of the substrate into the 20S proteasome for degradation.

Description

To further investigate the mechanism of the carborane-induced BSA degradation, MIC − BSA conjugate-transfected LNCaP was treated with MG-132, a proteasome inhibitor, and HLI-373, an E3 ligase inhibitor (Figure 5C,D). The degradation of MIC − BSA (3/1) and MIC − BSA (10/1) was suppressed in LNCaP treated with MG-132 or HLI-373, whereas the value of unmodified BSA transfected into LNCaP changed negligibly. These results indicate that the carborane conjugation of proteins could induce conjugated protein degradation via the 26S proteasome pathway involving ubiquitin E3 ligase. However, it has been reported that lysine residues of protein play an essential role in ubiquitination via E3 ligase.37 From the results shown in Figure 4B, the exposure of lysine residues on BSA was increased with the number of carboranes conjugated. Therefore, the carborane conjugation of proteins induced the exposure of lysine residues on the conjugated protein via microscopic conformational changes, resulting in degradation via the ubiquitin E3 ligase-related proteasome.