The sodium-independent iodide/chloride transporter is named as
Rephrasing the question:
What is the name of the sodium-independent iodide/chloride transporter?
Pendrin is the sodium-independent iodide/chloride transporter (option 2)
Let’s go through and discuss each of these proteins:
Megalin is also known as LRP2 (low density lipoprotein-related protein 2). It is a receptor found in the plasma membrane of many absorptive epithelial cells that binds to multiple ligands and mediates their endocytosis. Megalin is involved in proximal tubule protein uptake in the kidney, and serves as a receptor for the protein thyroglobulin in thyrocytes (epithelial cells in the thyroid).
Pendrin is an anion exchange protein, interesting as a sodium-independent chloride-iodide exchanger. It controls the flow of iodide into the thyroid gland. Mutations in this protein are associated with Pendred syndrome, the most common form of syndromic deafness, an autosomal-recessive disease. Pendred syndrome is characterized by thyroid goitre and enlargement of the vestibular aqueduct resulting in deafness. It also seems to have a role in airway hyperreactivity and inflammation, possibly due to pendrin’s effects on ion concentration in the fluid in the airways.
Transthyretin is a transport protein found in the serum and cerebrospinal fluid that transports thyroxine and retinol. The transthyretin in the blood is formed in the liver, and that in the cerebrospinal fluid is secreted by the choroid plexus. Transthyretin misfolding and aggregation is associated with amyloid diseases such as senile systemic amyloidosis, familial amyloid polyneuropathy, and familial amyloid cardiomyopathy. Considering this, it is not surprising that functional transthyretin also has a protective effect in Alzheimer’s disease.
Prestin is the motor protein of the outer hair cells of the inner ear of the mammalian cochlea. It is critical to sensitive hearing in mammals, and mutations in it are associated with non-syndromic hearing loss. Recently, anion transport has been shown to be an important mediator of prestin’s effect on the electromotility of hair cells.
Fong, P. Thyroid iodide efflux: a team effort? The Journal of Physiology, 589(24): 5929-5939.