Human homologue of the Drosophila discs large tumor suppressor protein forms an oligomer in solution -: Identification of the self-association site

The human homologue of the Drosophila discs large tumor suppressor protein (hDlg), a member of the membrane-associated guanylate kinase (MAGUK) superfamily, interacts with K+ channels, N-methyl-D-aspartate receptors, calcium ATPase, adenomatous polyposis coli, and PTEN tumor suppressor proteins, and several viral oncoproteins through its PDZ domains. MAGUKs play pivotal roles in the clustering and aggregation of receptors, ion channels, and cell adhesion molecules at the synapses. To investigate the physiological basis of hDlg interactions, we examined the self-association state of full-length hDlg as well as defined segments of hDlg expressed as recombinant proteins in bacteria and insect Sf9 cells. Gel permeation chromatography of full-length hDlg revealed that the purified protein migrates as a large particle of size >440 kDa, Similar measurements of defined domains of hDlg indicated that the anomalous mobility of hDlg originated from its amino-terminal domain. Ultrastructural analysis of hDlg by low angle rotary shadow electron microscopy revealed that the full-length hDlg protein as well as its amino-terminal domain exhibits a highly flexible irregular shape. Further evaluation of the self-association state of hDlg using sedimentation equilibrium centrifugation,, matrix-assisted laser desorption/ionization mass spectrometry, and chemical crosslinking techniques confirmed that the oligomerization site of hDlg is contained within its amino-terminal domain. This unique amino-terminal domain mediates multimerization of hDlg into dimeric and tetrameric species in solution, Sedimentation velocity experiments demonstrated that the oligomerization domain exists as an elongated tetramer in solution, In vitro mutagenesis was used to demonstrate that a single cysteine residue present in the oligomerization domain of hDlg is not required for its self-association. Understanding the oligomerization status of hDlg may help to explicate the mechanism of hDlg association with multimeric K+ channels and dimeric adenomatous polyposis coli tumor suppressor protein. Our findings, therefore, begin to rationalize the role of hDlg in the clustering of membrane channels and formation of multiprotein complexes necessary for signaling and cell proliferation pathways.