A 14-year-old girl presents with a 5-day history of hypertension, oliguria, and hematuria. She was seen 2 weeks earlier for a severe throat infection with group A (b-hemolytic) streptococci. A kidney biopsy displays glomerulonephritis. Immunofluorescence staining for which of the following proteins would provide the strongest evidence that this patient's glomerulonephritis is mediated by immune complexes?