Intrinsically disordered features of carbonic anhydrase IX proteoglycan-like domain

Abstract

hCA IX is a multi-domain protein belonging to the family of hCAs which are ubiquitous zinc enzymes that catalyze the reversible hydration of CO₂ to HCO₃^- and H⁺. hCA IX is a tumor-associated enzyme with a limited distribution in normal tissues, but over-expressed in many tumors, and is a promising drug target. Although many studies concerning the CA IX catalytic domain were performed, little is known about the proteoglycan-like (PG-like) domain of hCA IX which has been poorly investigated so far. Here we attempt to fill this gap by providing an overview on the functional, structural and therapeutic studies of the PG-like domain of hCA IX which represents a unique feature within the CA family. The main studies and recent advances concerning PG role in modulating hCA IX catalytic activity as well as in tumor spreading and migration are here reported. Special attention has been paid to the newly discovered disordered features of the PG domain which open new perspectives about its molecular mechanisms of action under physiological and pathological conditions, since disorder is likely involved in mediating interactions with partner proteins. The emerged disordered features of PG domain will be explored for putative diagnostic and therapeutic applications involving CA IX targeting in tumors.

Keywords: Carbonic anhydrase; Disease; Disorder; Hypoxic tumour; IDPs; IDRs; Natively unfolded.

Fig. 1

CA IX sequence analysis. a Schematic domain organization. The signal peptide, SP (residues 1–37, gray); the proteoglycan-like domain, PG (residues 53–111, blue); the catalytic domain, CA (residues 137–391, green); the transmembrane segment, TM (residues 415–433, yellow) and the intracytoplasmic tail, IC (residues 434–459, orange) are reported. PG_38-136, which is formed by PG domain and two short flanking sequences (white) is also displayed. b Amino acidic sequence of PG_38-136. The six repeats are underlined [26]

Fig. 2

Disorder features of hCA IX; for clarity, only the monomeric structure is shown. a Disorder analysis of PG_38-136 using two different predictors: PONDR-FIT (black line) and DISMETA (dashed line). Values greater than 0.5 indicate a propensity for disorder [26]. b Representative structure of hCA IX (38–391) obtained from Modeling and crystallographic strudies [26]. PG_38-136 and CA domain are displayed as red and gray cartoons, respectively

Fig. 3

Schematic representation of pH regulation process within tumor cell under hypoxic conditions. Proteins involved in the process are schematically reported: Na ⁺ /H ⁺ exchanger (NHE, light blue), sodium-bicarbonate co-transport (NBC, green), monocarboxylate transporter (MCT, purple), anion exchanger (AE, blue), V-type H⁺ ATPase (V-ATPase, orange), and carbonic anhydrase IX (CA IX, red)