**The TRIOBP Isoforms and Their Distinct Roles in Actin Stabilization, Deafness, Mental Illness, and Cancer**

### **Beti Zaharija** † **, Bobana Samardžija** † **and Nicholas J. Bradshaw \***

Department of Biotechnology, University of Rijeka, 51000 Rijeka, Croatia; beti.zaharija@biotech.uniri.hr (B.Z.); bobana.samardzija@biotech.uniri.hr (B.S.)

**\*** Correspondence: nicholas.b@uniri.hr

† These authors contributed equally to the manuscript.

Received: 16 September 2020; Accepted: 26 October 2020; Published: 27 October 2020

**Abstract:** The *TRIOBP* (*TRIO* and *F-actin Binding Protein*) gene encodes multiple proteins, which together play crucial roles in modulating the assembly of the actin cytoskeleton. Splicing of the *TRIOBP* gene is complex, with the two most studied TRIOBP protein isoforms sharing no overlapping amino acid sequence with each other. TRIOBP-1 (also known as TARA or TAP68) is a mainly structured protein that is ubiquitously expressed and binds to F-actin, preventing its depolymerization. It has been shown to be important for many processes including in the cell cycle, adhesion junctions, and neuronal differentiation. TRIOBP-1 has been implicated in schizophrenia through the formation of protein aggregates in the brain. In contrast, TRIOBP-4 is an entirely disordered protein with a highly specialized expression pattern. It is known to be crucial for the bundling of actin in the stereocilia of the inner ear, with mutations in it causing severe or profound hearing loss. Both of these isoforms are implicated in cancer. Additional longer isoforms of TRIOBP exist, which overlap with both TRIOBP-1 and 4. These appear to participate in the functions of both shorter isoforms, while also possessing unique functions in the inner ear. In this review, the structures and functions of all of these isoforms are discussed, with a view to understanding how they operate, both alone and in combination, to modulate actin and their consequences for human illness.

**Keywords:** TRIOBP; cancer; deafness; hearing loss; mental illness; schizophrenia; actin; cytoskeleton; disordered structure; protein aggregation
