Modeling and experimental validation of the binary complex of the plectin actin-binding domain and the first pair of fibronectin type III (FNIII) domains of the β4 integrin

Abstract

The binding of plectin to the β4 subunit of the α6β4 integrin is a critical step in the formation of hemidesmosomes. An important interaction between these two proteins occurs between the actin-binding domain (ABD) of plectin and the first pair of fibronectin type III (FNIII) domains and a small part of the connecting segment of β4. Previously, a few amino acids, critical for this interaction, were identified in both plectin and β4 and mapped on the crystal structures of the ABD of plectin and the first pair of FNIII domains of β4. In the present study, we used this biochemical information and protein-protein docking calculations to construct a model of the binary complex between these two protein domains. The top scoring computational model predicts that the calponin-homology 1 (CH1) domain of the ABD associates with the first and the second FNIII domains of β4. Our mutational analysis of the residues at the proposed interface of both the FNIII and the CH1 domains is in agreement with the suggested interaction model. Computational simulations to predict protein motions suggest that the exact model of FNIII and plectin CH1 interaction might well differ in detail from the suggested model due to the conformational plasticity of the FNIII domains, which might lead to a closely related but different mode of interaction with the plectin-ABD. Furthermore, we show that Ser-1325 in the connecting segment of β4 appears to be essential for the recruitment of plectin into hemidesmosomes in vivo. This is consistent with the proposed model and previously published mutational data. In conclusion, our data support a model in which the CH1 domain of the plectin-ABD associates with the groove between the two FNIII domains of β4.