Transcriptional activator involved in ER stress responses. Functions as a stress sensor and transducer in ER stress signaling pathway. After proteolysis by SBT6.1 (S1P) and S2P, the N-terminal bZIP component is translocated to the nucleus, where it activates the expression and production of ER chaperones (PubMed:18156219, PubMed:18634751, PubMed:18849477, PubMed:20876872). Following ER stress, activates proteins involved in brassinosteroid (BR) signaling, which is required for stress acclimation and growth (PubMed:20876872). Homodimer. Hetereodimers with BZIP17, BZIP49 or BZIP60. Binds the CCAAT box-binding heterotrimeric factor composed of NFYA4, NFYB3 and NFYC2 (PubMed:20207753). Interacts with SAR1B and STL2P (PubMed:22335396). Interacts (via C-terminus) with MED37A/BIP1 and MED37B/BIP3. MED37A/BIP1 and MED37B/BIP3 dissociates from BZIP28 under ER stress (PubMed:23624714). Translocates to the nucleus following heat shock or tunicamycin treatment (ER stress) (PubMed:18156219, PubMed:18634751, PubMed:18849477, PubMed:20876872, PubMed:22335396, PubMed:23558471, PubMed:23624714). Relocation of BZIP28 from ER to the nucleus occurs through the Golgi and is S2P-dependent (PubMed:20876872, PubMed:22335396, PubMed:23558471, PubMed:23624714). By heat shock. Belongs to the bZIP family.