The GTD-binding domain is a plant-specific protein-protein interaction domain. It emerged in primitive land plants and founded a multigene family that is conserved in all flowering plants. Proteins with GTD-binding domains fall into four groups, where group 1-3 contain the GTD-binding domain at the C-terminal half of the protein and one (group 2) or more (group1) predicted transmembrane domains, or an endoplasmic reticulum signal peptide (group 3) at the N-terminus, whereas group 4 contains the GTD-binding domain near the N-terminus. GTD-binding domain proteins may constitute a family of myosin receptors, which are associated with the surface of specific plant organelles, bind to the globular tail domain (GTD) of myosin motor proteins, and thereby promote actin-dependent organelle motility. It seems likely that myosin binding is a common property of the GTD-binding domain, whereas the ability of FLOURY1 to bind maize-specific zeins is a specific feature of this endoplasmic reticulum (ER)-associated protein.

The GTD-binding domain is predicted to adopt a coiled-coil structure.

Some proteins known to contain a GTD-binding domain are listed below:

• Maize FLOURY1 (FL1), which belongs to group 1. Its GTD-binding domain facilitates the localization of 22-kD α-zein [1].
• Arabidopsis myosin binding (MyoB) proteins 1-6 and 7, which belong respectively to group 3 and 4. They bind to myosin XI [2].
• Tobacco RAC5 interacting subapical pollen tube protein (RISAP), which belongs to group 3. It binds via its GTD-binding domain to the GTD domain of a pollen tube myosin XI [3].
• Lily LLP13, which belongs to group 3. It is likely a cytoskeleton-binding protein that binds with intermediate filaments (IFs) that potentially exist in pollen tubes [4].

The profile we developed covers the entire GTD-binding domain.