The norepinephrine transporter (NET) mediates reuptake of norepinephrine 
released from neurons, and, as such, it is an important regulator of 
noradrenergic neurotransmission. Recently, our laboratory reported a 
polymorphism in the human NET (hNET) gene A457P in an individual with the 
autonomic disorder orthostatic intolerance (OI). The presence of the hNET-A457P 
allele tracked with elevated heart rates and plasma NE levels in family members. 
hNET-A457P lacks >98% transport activity in several heterologous expression 
systems. In the present work, Western blot and biotinylation analyses performed 
in transiently transfected COS-7 cells revealed impairment in processing of 
hNET-A457P to the fully glycosylated form and a decrease in surface expression 
to approximately 30% of hNET-wild type (hNET-wt). Because the hNET-A457P 
mutation is carried on a single allele in OI subjects, we examined the influence 
of cotransfection of hNET-wt and hNET-A457P and found that hNET-A457P exerts a 
dominant-negative effect on hNET-wt uptake activity. Experiments to determine 
oligomerization as a potential mechanism of the dominant-negative effect 
demonstrated that hNET-A457P coimmunoprecipitates with, and diminishes surface 
expression of, hNET-wt. These results reveal that hNET-A457P causes a 
conformational disruption that interferes with transporter biosynthetic 
progression and trafficking of both the mutant transporter and hNET-wt. These 
results elucidate a molecular mechanism for the disrupted NE homeostasis and 
cardiovascular function evident in OI patients with the hNET-A457P mutation.