The present invention provides isolated GPBP-interacting 90 and 130 kDa polypeptides, and portions thereof (GIP90/130 polypeptides), antibodies to the GIP90/130 polypeptides, and pharmaceutical compositions thereof. The present invention also provides isolated GIP90/130 nucleic acid sequences, expression vectors comprising the nucleic acid sequences, and host cells transfected with the expression vectors. The invention further provides methods for detecting the GIP90/130 polypeptides or nucleic acid sequences, methods for inhibiting interactions between GPBP and GIP90/130 polypeptides, between pol k76 and GIP90/130 polypeptides or aggregation of GIP90/130 polypeptides, and methods for treating patients with autoimmune disorders or cancer.