MSP430F4351: GPIO status

The LPM modes just disable certain clocks. But the GPIOs do not use a clock; section 2.3 of the User's Guide says:

Peripherals operating with any disabled clock are disabled until the clock becomes active. The peripherals may also be disabled with their individual control register settings. All I/O port pins and RAM/registers are unchanged.

The I²C module can be used in LPM modes that do not disable the clock that it is using. Furthermore, section 21.3.6 of the User's Guide says:

Using the USCI Module in I²C Mode With Low-Power Modes

The USCI module provides automatic clock activation for SMCLK for use with low-power modes. When SMCLK is the USCI clock source, and is inactive because the device is in a low-power mode, the USCI module automatically activates it when needed, regardless of the control-bit settings for the clock source. The clock remains active until the USCI module returns to its idle condition. After the USCI module returns to the idle condition, control of the clock source reverts to the settings of its control bits. Automatic clock activation is not provided for ACLK.

In other words, if you start an I²C transaction and then immediately enter LPM, the hardware will keep SMCLK active until the transaction has finished.

Additionally:

In I²C slave mode no internal clock source is required because the clock is provided by the external master. It is possible to operate the USCI in I²C slave mode while the device is in LPM4 and all internal clock sources are disabled. The receive or transmit interrupts can wake up the CPU from any low power mode.