FC0110
Knob-associated Histidine-rich Protein (KAHRP)  -  Spectrin

Biological function
KAHRP, together with Erythrocyte Membrane Protein 1 (EMP1) cluster on protrusions of Plasmodium falciparum-infected erythrocyte surface. These knobs enable cytoadherence into the host microvasculature.

Domain organization/sequence features
The N-terminal half, referred to as K1 (38-362 AA), includes the eponymous histidine-rich region and fragments therein have been demonstrated to associate with the erythrocyte membrane and ankyrin. The C-terminal half of KAHRP, divided into K2 (356-533 AA) and K3 (533-654 AA) segments, comprises two amino acid sequence repeat elements (5 ́ and 3 ́). The KAHRP C-terminal half and fragments therein also localize to the erythrocyte periphery and associate with spectrin, a multi-domain protein primarily composed of triple helical bundles.

Structural evidence
Fluorescence polarisation (FP)-monitored titrations showed binding between spectrin β10–14 and the 5’ repeat element of KAHRP. NMR spectra of KAHRP 5’-repeat confirmed the disordered state of this segment, and the involvement of many KAHRP amino acids in the spectrin complex. MD simulations suggested that KAHRP forms a dynamic complex on the spectrin surface.

Biochemical evidence
Gradual truncations of the KAHRP 5 ́ repeat, reduced β10–14 binding proportional to the number of repeat elements eliminated. Deletion of one or more β spectrin domains also resulted in step-wise reduction of K2 affinity.

Structure/Mechanism
Binding is driven by electrostatic interactions. Docking models show interactions between charged KAHRP residues and complementary charged clusters of spectrin. Interestingly, the MD simulations also evidence the dynamic behavior in the KAHRP–spectrin complex as demonstrated by small changes in the binding conformation. Owing to non-specific electrostatic contacts, binding between β10–14 and the KAHRP 5 ́ repeat enhanced only by 2- to 5-fold by sequence specific interactions. This is comparable to the overall margin of specificity observed for KAHRP binding to spectrin fragments.

Mechanism category
tethering

Significance
Fuzziness of the complex weakens sequence requirements for binding and enable gradual changes in affinity.

Submitted by
Ioannis Vakonakis   ioannis.vakonakis@bioch.ox.ac.uk