Thymosin Beta-4 (Full Protein)

Thymosin Beta-4 (Tβ4) is a highly conserved 43-amino acid G-actin sequestering protein that is ubiquitously expressed in mammalian cells. It is one of the most abundant intracellular peptides in eukaryotic cells, found at concentrations of 200-500 μM in platelets and approximately 50 μM in most other cells.

Tβ4 was originally identified as a thymic hormone in 1981 by Goldstein et al., but is now understood to be a fundamental regulator of the actin cytoskeleton in virtually all cell types, not just thymic cells.

**Key structure-function relationships:** Tβ4 contains three functional domains: 1. **N-terminal domain (aa 1-4)**: Anti-inflammatory, anti-apoptotic 2. **LKKT actin-binding domain (aa 17-23)**: G-actin sequestration, cell migration promotion — **this is the TB-500 bioactive fragment** 3. **C-terminal domain (aa 24-43)**: Additional binding interactions, cardiac effects

The relationship to TB-500: TB-500 is a synthetic 17-amino acid peptide corresponding to the LKKT domain of Tβ4. TB-500 is more widely available as a research tool because it is easier to synthesize at scale. However, the full Tβ4 protein has pharmacological properties beyond what the LKKT fragment alone provides.

**Full Tβ4 additional properties not replicated by TB-500:** - **Cardiac repair**: Tβ4 activates cardiac progenitor cells (epicardial-derived Wt1+ cells) to migrate and differentiate into new cardiomyocytes, coronary smooth muscle, and endothelial cells. This was demonstrated in Bock-Marquette's landmark 2004 Nature paper showing Tβ4 promotes survival and cardiac repair post-MI. - **Hair follicle activation**: Full Tβ4 (not the LKKT fragment) activates hair follicle stem cells to re-enter the growth (anagen) phase — demonstrated in Safer et al. (2008). - **Ocular healing**: Tβ4 promotes corneal epithelial healing via a distinct mechanism from simple actin dynamics (tear film stabilization, stem cell activation in limbal region).

RegeneRx Biopharmaceuticals has the most extensive clinical development program for Tβ4, conducting Phase II trials for corneal wound healing (RGN-259), dermal wound healing, and cardiac repair.