# Oligomerization Function of the Native Exon 5 Sequence of Ameloblastin Fused with Calmodulin

## TL;DR

Researchers engineered a protein by fusing a disordered ameloblastin exon with calmodulin, enabling self-assembly under specific conditions.

## Contribution

Demonstrated that the oligomerization function of ameloblastin exon 5 can be transferred to a new protein context.

## Key findings

- The engineered eCaM protein self-assembles into oligomers in a concentration- and time-dependent manner.
- Oligomerization of eCaM is reversible upon dilution.
- A key residue mutation in exon 5 abolished self-assembly, confirming its role in oligomerization.

## Abstract

The evolution of
proteins is primarily driven by the
combinatorial
assembly of a limited set of pre-existing modules known as protein
domains. This modular architecture not only supports the diversity
of natural proteins but also provides a robust strategy for protein
engineering, enabling the design of artificial proteins with enhanced
or novel functions for various industrial applications. Among these
functions, oligomerization plays a crucial role in enhancing protein
activity, such as by increasing the binding capacity of antibodies.
To investigate the potential of engineering oligomerization, we examined
the transferability of the sequence domain encoded by exon 5 (Ex5),
which was originally responsible for the oligomerization of ameloblastin
(AMBN). We designed a two-domain protein composed of Ex5 in combination
with a monomeric, globular, and highly stable protein, specifically
calmodulin (CaM). CaM represents the opposite protein character to
AMBN, which is highly disordered and has a dynamic character. This
engineered protein, termed eCaM, successfully acquired an oligomeric
function, inducing self-assembly under specific conditions. Biochemical
and biophysical analyses revealed that the oligomerization of eCaM
is both concentration- and time-dependent, with the process being
reversible upon dilution. Furthermore, mutating a key oligomerization
residue within Ex5 abolished the self-assembly of eCaM, confirming
the essential role of the Ex5 motif in driving oligomerization. Our
findings demonstrate that the oligomerization properties encoded by
Ex5 can be effectively transferred to a new protein context, though
the positioning of Ex5 within the protein structure is critical. This
work highlights the potential of enhancing monomeric proteins with
oligomeric functions, paving the way for industrial applications and
the development of proteins with tailored properties.