The beta-appendages of the four adaptor-protein (AP) complexes: structure and binding properties, and identification of sorting nexin 9 as an accessory protein to AP-2

Biochem J. 2002 Mar 15;362(Pt 3):597-607. doi: 10.1042/0264-6021:3620597.


Adaptor protein (AP) complexes are essential components for the formation of coated vesicles and the recognition of cargo proteins for intracellular transport. Each AP complex exposes two appendage domains with that function to bind regulatory accessory proteins in the cytosol. Secondary structure predictions, sequence alignments and CD spectroscopy were used to relate the beta-appendages of all human AP complexes to the previously published crystal structure of AP-2. The results suggested that the beta-appendages of AP-1, AP-2 and AP-3 have similar structures, consisting of two subdomains, whereas that of AP-4 lacks the inner subdomain. Pull-down and overlay assays showed partial overlap in the binding specificities of the beta-appendages of AP-1 and AP-2, whereas the corresponding domain of AP-3 displayed a unique binding pattern. That AP-4 may have a truncated, non-functional domain was indicated by its apparent inability to bind any proteins from cytosol. Of several novel beta-appendage-binding proteins detected, one that had affinity exclusively for AP-2 was identified as sorting nexin 9 (SNX9). SNX9, which contains a phox and an Src homology 3 domain, was found in large complexes and was at least partially associated with AP-2 in the cytosol. SNX9 may function to assist AP-2 in its role at the plasma membrane.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptor Protein Complex 3
  • Adaptor Protein Complex alpha Subunits
  • Adaptor Protein Complex delta Subunits
  • Amino Acid Sequence
  • Binding Sites
  • Carrier Proteins / chemistry
  • Carrier Proteins / metabolism*
  • Cell Membrane / metabolism
  • Circular Dichroism
  • Conserved Sequence
  • Cytosol / metabolism
  • Glutathione Transferase / metabolism
  • Humans
  • Membrane Proteins / chemistry
  • Membrane Proteins / metabolism*
  • Molecular Sequence Data
  • Protein Conformation
  • Protein Structure, Secondary
  • Protein Transport
  • Recombinant Fusion Proteins / chemistry
  • Recombinant Fusion Proteins / metabolism
  • Sequence Alignment
  • Sorting Nexins
  • Transcription Factor AP-1 / chemistry
  • Transcription Factor AP-1 / metabolism*
  • Transcription Factors / chemistry
  • Transcription Factors / metabolism*
  • Vesicular Transport Proteins


  • AP3D1 protein, human
  • Adaptor Protein Complex 3
  • Adaptor Protein Complex alpha Subunits
  • Adaptor Protein Complex delta Subunits
  • Carrier Proteins
  • Membrane Proteins
  • Recombinant Fusion Proteins
  • SNX9 protein, human
  • Sorting Nexins
  • Transcription Factor AP-1
  • Transcription Factors
  • Vesicular Transport Proteins
  • Glutathione Transferase