In molecular biology, the Cbl TKB domain (tyrosine kinase

Tyrosine kinase

A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to a protein in a cell. It functions as an "on" or "off" switch in many cellular functions....

 binding domain), also known as the phosphotyrosine binding (PTB) domain is a conserved region found at the N-terminus of Cbl adaptor protein

Adaptor protein

Signal transducing adaptor proteins are proteins which are accessory to main proteins in a signal transduction pathway. These proteins tend to lack any intrinsic enzymatic activity themselves but instead mediate specific protein–protein interactions that drive the formation of protein complexes...

s. This N-terminal region is composed of three evolutionarily conserved domains

Protein domain

A protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...

: an N-terminal four-helix bundle domain, an EF hand

EF hand

The EF hand is a helix-loop-helix structural domain found in a large family of calcium-binding proteins. The EF-hand motif contains a helix-loop-helix topology, much like the spread thumb and forefinger of the human hand, in which the Ca2+ ions are coordinated by ligands within the loop...

-like domain and a SH2

SH2 domain

The SH2 domain is a structurally conserved protein domain contained within the Src oncoprotein and in many other intracellular signal-transducing proteins...

-like domain, which together are known to bind to phosphorylated tyrosine residues.