Biophysical Characterization of Cancer-Related Carbonic Anhydrase IX

Upregulation of carbonic anhydrase IX (CA IX) is associated with several aggressive forms of cancer and promotes metastasis. CA IX is normally constitutively expressed at low levels in selective tissues associated with the gastrointestinal tract, but is significantly upregulated upon hypoxia in cancer. CA IX is a multi-domain protein, consisting of a cytoplasmic region, a single-spanning transmembrane helix, an extracellular CA catalytic domain, and a proteoglycan-like (PG) domain. Considering the important role of CA IX in cancer progression and the presence of the unique PG domain, little information about the PG domain is known. Here, we report biophysical characterization studies to further our knowledge of CA IX. We report the 1.5 Å resolution crystal structure of the wild-type catalytic domain of CA IX as well as small angle X-ray scattering and mass spectrometry of the entire extracellular region. We used matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry to characterize the spontaneous degradation of the CA IX PG domain and confirm that it is only the CA IX catalytic domain that forms crystals. Small angle X-ray scattering analysis of the intact protein indicates that the PG domain is not randomly distributed and adopts a compact distribution of shapes in solution. The observed dynamics of the extracellular domain of CA IX could have physiological relevance, including observed cleavage and shedding of the PG domain.