Dissecting interactions within focal adhesions: studies on vinculin

Abstract

Vinculin and paxillin are proteins that localise to focal adhesions. Focal adhesions are
specialised sites of cell attachment between the cytoplasmic side of the cell membrane and
the extracellular matrix. They consist of receptors that link extracellular matrix ligands to
the actin cytoskeleton via various protein assemblies. They act as mechanical links and as
sites of signal transduction to transduce signals for cell locomotion, cell attachment and
detachment, apoptosis and gene expression. A range of cellular responses depend critically
on the composition and regulation of focal adhesions.

Vinculin and paxillin will interact in vitro. The tail domain of vinculin interacts with a
motif on paxillin called an LD motif. The interaction between LD motifs and their target
proteins is important for regulation of focal adhesion signalling, yet little is known
regarding recognition mechanisms between LD domains and interaction partners.

In this thesis, the molecular nature of the vinculin tail (Vt) and paxillin LD motif
interaction has been studied using purified Vt, synthesised LD peptide mimics and
recombinant paxillin His-LD1/LD2. The 1H-15N-HSQC spectrum of a Vt/I997S mutant
has been assigned and the NH assignments transferred to a wild-type Vt spectrum.
Chemical shift perturbation studies have subsequently been undertaken using wild-type Vt
and paxillin.

The data presented here is consistent with specificity for LD motifs and points to an
interaction between Vt and LD1 and LD2. In contrast there is little or no interaction with
LD4. Two binding models are proposed; a single binding site on Vt face 3-4 where LD
motifs bind in an extended conformation, or a two-site binding on Vt face 3-4 with two
LD motifs bound as a-helices. Both models need further analysis. NMR data for the
paxillin His-LD1/LD2 construct suggests a predominantly unstructured molecule in
solution that can catalyse precipitation of Vt when Vt is added to excess.

It is clear that the mechanism of interaction between Vt and paxillin LD motifs is distinct
from that of focal adhesion kinase interacting with paxillin. Further investigation is
required to elucidate the precise mechanism of binding. A comparison of this data with
other LD-protein interactions suggests there are little similarities between the target
sequences that LD motifs recognise and that target proteins can be structurally different.

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Identifiers

ORCID for Jorn Werner: orcid.org/0000-0002-4712-1833

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