We report the molecular, genetic and functional analysis of a case of thrombasthenic phenotype. The proband showed absence of platelet glycoprotein (GP)IIb and very low content of GPIIIa, and both his parents showed a marked reduction in the levels of platelet GPIIb-IIIa. Single-stranded conformational polymorphism-polymerase chain reaction (SSCP-PCR) analysis and direct sequencing of PCR-amplified GPIIb exon-12 revealed the presence of a G-->A transition at position 1063 with the expected substitution of glutamate 324 with lysine (K). This mutation did not alter the level of GPIIb mRNA. Co-expression of normal or mutant [324K] GPIIb with normal human GPIIIa in Chinese hamster ovary (CHO) cells failed to show surface exposure of [324K]GPIIb-IIIa complexes. Pulse-chase and immunoprecipitation analysis demonstrated that [324K]GPIIb cDNA was translated into proGPIIb, but neither mutant GPIIb heavy chain (GPIIbH) nor [324K]GPIIb-GPIIIa complexes were detected, suggesting that this mutation is the underlying molecular basis for the thrombasthenic phenotype. Mutation analysis demonstrated that 324E of GPIIb could be replaced by other negatively charged or polar amino acids (AAs) without impairing the surface expression of GPIIb-IIIa. However, substitution of 324E of GPIIb for a positively charged AA other than K prevented the expression of GPIIb-IIIa complexes. These observations suggest that a domain encompassing 324E of GPIIb is essential for heterodimerization with GPIIIa and its substitution for a positively charged residue precludes normal subunit association.