The ltk gene encodes a novel receptor-type protein tyrosine kinase

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Previous studies suggested a unique structure for the ltk tyrosine kinase, with a minimal extracellular domain and potential initiation at a CTG codon.
• These earlier findings implied an atypical function for the ltk protein among tyrosine kinases.

Purpose of the Study:

• To clone and sequence a full-length human ltk cDNA.
• To characterize the ltk gene product's structure and function.
• To clarify the translational initiation site and protein characteristics of ltk.

Main Methods:

• Cloning and sequencing of a full-length human ltk cDNA.
• Ribonuclease protection analysis to determine mRNA abundance.
• In vitro transcription and translation assays.
• COS-1 cell transfection to assess protein expression, glycosylation, and kinase activity.

Main Results:

• A novel, full-length human ltk cDNA was identified, encoding a protein with typical receptor-type tyrosine kinase features.
• The encoded protein possesses an ATG initiation codon, a signal sequence, and a substantial extracellular domain (347 amino acids).
• Experimental data confirmed initiation at the ATG codon, producing a 100 kDa glycosylated protein with in vitro kinase activity, and indicated this cDNA represents the most abundant ltk mRNA species.

Conclusions:

• The ltk gene product is a functional, cell surface receptor-type protein tyrosine kinase.
• The identified ltk cDNA and its encoded protein product are consistent with typical receptor tyrosine kinase characteristics.
• The ltk protein likely plays a role in cellular signaling as a receptor for an as-yet-unidentified growth factor.