The biosynthesis and post-translational modifications of plasma proteins have been investigated in cultured rat hepatodytes.
1. Haptoglobin: Haptoglobin is initially synthesized as a single polypeptide precursor with Mr 42,000, which is immediately cleaved into the <alpha> (9,500) and <beta> (33,000) subunits in the endoplasmic reticulum. During the intracellular teransport,the <beta> subunit which has high-mannose type oligosaccharides is converted to a mature form (36,000) with the complex type oligosaccharides. Structural analyses demonstrate that <beta> subunit has four oligosaccharide chains with two giantennary and two triantennary complex type chains.
2. Complement C3: C3 is initially synthesized as a single polypeptide precursor with Mr 180,000, which is proteolytically processed to the <alpha> (115,000) and <beta> (65,000) at the trans Golgi region before the sectretion. The <alpha> subnit has three oligosaccharides consisting of one each of biantennary and triantennary complex type chains and one high-mannose type chain.
3. Fibrinogen: Of the three subunits (A<alpha>,B<beta> and <gamma>) of rat fibrinogen,the synthesis of the A<alpha> chain is sihnificantly slower than that of the two others,and considered to be a rate-limiting factor in assembly and secretion of the fibrinogen molecule. The variant <gamma>B chain of fibrinogen is sulfated on the fyrosine residue at the second position from the C-terminus,Which occurs at the trans Golgi region.